LIBRARY 

OF  THE 

UNIVERSITY  OF  CALIFORNIA. 


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LIBRARY 

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HOME  UNIVERSITY  LIBRARY 
OF  MODERN  KNOWLEDGE 

No.  12 


Editors : 

HERBERT    FISHER,  M.A.,  F.B.A. 
PROF.  GILBERT  MURRAY,  LiTT.D., 

LL.D.,  F.B.A. 

PROF.  J.  ARTHUR    THOMSON,  M.A. 
PROF.  WILLIAM  T.  BREWSTER,  M.A. 


THE   HOME    UNIVERSITY  LIBRARY 
OF   MODERN  KNOWLEDGE 

VOLUMES  NOW  READY 

HISTORY  OF  WAR  AND  PEACE     .    G.  H.  FERRIS 

POLAR  EXPLORATION DR.W.8.BRUCE,LL.D.,F.R.S.E. 

THE  FRENCH  REVOLUTION   .    .     .    HILAIRE  BELLOC,  M.  P. 

THE   STOCK  EXCHANGE :  A  SHORT 
STUDY  OF  INVESTMENT  AND  SPECULATION    F.  W.  HIRST 

IRISH  NATIONALITY ALICE  STOPFORD  GREEN 

THE  SOCIALIST  MOVEMENT  .    .    .    J.  RAMSAY  MACDONALD,  M.P. 

PARLIAMENT :  ITS  HISTORY,  CONSTITU- 
TION, AND  PRACTICE SIR  COURTENAY  ILBERT,  K.C.B., 

K.C.S.I. 

MODERN  GEOGRAPHY MARION    I.    NEWBIGIN,  D.S.C. 

(Lond.) 

WILLIAM  SHAKESPEARE     ....    JOHN  MASEFIELD. 

THE  EVOLUTION  OF  PLANTS    .    .    D.H.ScorT,M.A.,LL.D.,F.R.S. 

VOLUMES  READY  IN  JULY 

THE  OPENING-UP  OF  AFRICA    .    .    Bra  H.  H.  JOHNSTON,  G.C.M.G., 

K.C.B.,  D.Sc.,  F.Z.S. 

MEDIEVAL  EUROPE H.  W.  C.  DAVIS,  M.A. 

MOHAMMEDANISM D.    8.    MAHGOLIOUTH,    M.A., 

D.LlTT. 

THE  SCIENCE  OF  WEALTH     .     .     .    J.  A.  HOBSON,  M.A. 

HEALTH  AND  DISEASE W.  LESLIE  MACKENZIE,  M.D. 

INTRODUCTION  TO  MATHEMATICS    A.  N.  WHITBHBAD,  Sc.D.  F.R.S. 

THE  ANIMAL  WORLD F.  W.  GAMBLE,  D.Sc.,  F.R.S. 

EVOLUTION J.  ARTHUR  THOMSON,  M.A.,  and 

PATRICK  GEDDES,  M.A. 

LIBERALISM L.  T.  HOBHOUSE,  M.A. 

CRIME  AND  INSANITY DR.  C.  A.  MERCIER,  F.R.C.P., 

F.R.C.S. 

*#*  Other  volumes  in  active  preparation 


THE 
ANIMAL  WORLD 


BY 


F.  W.  GAMBLE,  F.R.S. 

PROFESSOR    OF    ZOOLOGY    IN    THE    UNIVERSITY    OF 
BIRMINGHAM 


WITH    INTRODUCTION 

BY 

SIR   OLIVER   LODGE,  F.R.S. 


NEW  YORK 
HENRY   HOLT  AND    COMPANY 

LONDON 
WILLIAMS  AND    NORGATE 


BIOLOGY 
LIBRARY 

G 


COPYRIGHT,  1911, 


HENRY    HOLT  AND    COMPANY 


THE  UNIVERSITY   PRESS,    CAMBRIDGE,  U.S.A. 


CONTENTS 

CHAP.  PAGE 

INTRODUCTION  BY  SIR  OLIVER  LODGE,  F.  R.  S.  .  vii 

I    THE  STRUCTURE  AND  CLASSIFICATION  OF  ANIMALS  13 

II    THE  MOVEMENTS,  SUCCESSION,  AND  DISTRIBUTION 

OF  ANIMALS 42 

III  THE  QUEST  FOR  FOOD 70 

IV  How  ANIMALS  BREATHE 87 

V    THE  COLOURS  OF  ANIMALS 121 

VI    THE  SENSES  OF  ANIMALS 141 

VII    SOCIETIES  AND  ASSOCIATIONS:  SYMBIOSIS    .     .     .  155 

VIII    THE  CARE  OF  THE  YOUNG 173 

IX     LlFE-HlSTORIES   OF   ANIMALS     .......  205 

X    HEREDITY  AND  VARIATION 230 

BIBLIOGRAPHY 253 

GLOSSARY-INDEX 255 


223162 


AUTHOR'S  PREFACE 

I  AM  especially  indebted  to  my  wife  for  help  in 
the  preparation  of  the  MS.  of  this  work. 

F.  W.  G. 


VI 


INTRODUCTION 

THE  mystery  of  life  in  its  interaction  with  or 
utilization  of  matter  is  one  of  the  outstanding 
puzzles  of  the  Universe  which  confronts  not 
only  every  physicist  and  chemist,  but  every 
educated  man.  Biologists,  who  know  so  much 
about  living  beings,  must  feel  the  deciphering 
of  the  hidden  meaning  of  life  itself  as  a  stand- 
ing challenge.  Every  year,  no  doubt,  brings 
them  nearer  to  a  solution,  but  to  all  appearance 
that  solution  is  still  far  away.  In  that  respect 
they  are  only  in  the  predicament  of  the  gravi- 
tational astronomer,  who,  though  able  to  apply 
his  theory  to  the  most  hidden  perturbation  and 
announce  predictions  which  are  capable  of  tri- 
umphant vindication,  yet  is  ignorant,  completely 
ignorant,  of  the  nature  of  the  gravitational  force 
itself.  From  time  to  time  thinkers  are  at  work, 
however,  and  every  decade  the  problem  shows 
signs  of  becoming  more  soluble:  hope  is  in  the 
air  as  regards  gravitation,  and  there  is  at  least 
vivid  speculation  regarding  life. 

Meanwhile,  the  mass  of  the  public  are  far  more 

interested  in  the  problems  presented   by  living 

creatures  than  in  those  pertaining  to  astronomical 

physics;  and  the  kinds  of    study  necessary  to 

vii 


viii  INTRODUCTION 

assimilate  the  researches  of  biologists,  so  long 
as  they  are  not  clothed  in  too  artificial  and  re- 
pellent a  language,  is  not  of  a  recondite  character, 
but  is  interesting  and  attractive. 

The  multifariousness  of  existence  is  so  great, 
however,  that  the  untrained  observer  must 
assuredly  lose  his  way  without  a  guide.  A  clue 
is  necessary,  one  which  will  call  attention  to  the 
points  of  resemblance,  the  similarity  amid  differ- 
ence, the  family  relationship  running  through 
the  animal  kingdom,  the  chain  or  intermediate 
stages  linking  together  the  simple  and  the  com- 
plex; a  student  can  thus  be  led  to  appreciate  the 
fundamental  properties  and  powers  which  are 
common  to  them  all,  but  which  put  on  so  differ- 
ent an  appearance  in  different  cases,  exhausting 
almost  every  possibility  of  variation,  and  ex- 
ceeding in  actual  achievement  the  resource  of 
the  most  fertile  imaginations. 

Most  textbooks  of  zoology  are  content  to  de- 
scribe the  different  organisms  in  an  ascending 
series,  beginning  with  a  single  cell  so  simple  as  to 
be  indiscriminately  animal  or  plant  or  neither 
or  both,  and  passing  on  through  a  definite  but 
many-branching  series  of  gradations  till  the  outer- 
most twigs  of  each  branch  are  reached — ferns, 
shall  we  say,  along  one  limb,  trees  and  flowering 
plants  along  another,  cuttlefish  along  another, 
bees  yet  another;  while,  following  other  lines  of 
development,  we  are  led  to  the  extraordinary 


INTRODUCTION  ix 

wealth  of  animal  life  culminating  for  a  time  re- 
spectively in  shark,  lizard,  eagle,  man. 

That  series  of  ascending  chapters  is  the  ordi- 
nary and  very  necessary  prelude  to  a  study  of 
zoology,  but  this  little  book  of  my  friend  and 
colleague,  Professor  Gamble,  pursues  a  different 
course. 

It  assumes  some  knowledge  of  the  animal 
kingdom  as  popularly  treated,  and  proceeds  to 
consider  it  not  so  much  as  a  chain  of  develop- 
ment, or  as  groups  to  be  subdivided  and  classified, 
but  from  the  point  of  view  of  function.  Its 
object  is  to  trace  the  similarity  of  function  run- 
ning through  the  whole  series,  to  emphasize  the 
extraordinarily  various  modes  in  which  these 
functions  are  performed,  the  diverse  organs 
grown  for  their  due  performance,  the  reaction  of 
achievement  on  form  and  of  form  on  achieve- 
ment— so  that  the  necessity  of  a  function  seems 
to  grow  a  suitable  organ  and  the  possession  of 
the  organ  leads  to  other  and  higher  functions — 
a  kind  of  action  and  reaction  of  the  utmost  inter- 
est and  variety.  In  this  way  the  similarity  of 
the  needs  of  the  highest  and  of  the  lowest — the 
likeness,  we  may  say,  of  man  to  the  Amreba — is 
forcibly  brought  out,  and  all  the  wealth  of  knowl- 
edge of  a  biologist  is  made  available  to  the  reader 
of  keen  aptitude  but  small  knowledge. 

For  the  thoughtful  and  philosophically-minded 
student  at  the  present  day,  such  a  book  is  most 


x  INTRODUCTION 

timely  and  helpful.  In  it  the  salient  facts  are 
dissected  out  from  an  overwhelming  mass  of 
material,  and  attention  concentrated  on  a  few 
functions  of  overwhelming  importance  and  on 
the  variety  of  ways  in  which  they  are  performed. 

Movement  or  locomotion  is  one  such  function, 
the  influence  of  which  on  the  life  of  the  race  can 
hardly  be  exaggerated.  Movement  at  some 
period  of  life  is  essential  to  both  animals  and 
plants,  and  the  power  entails  the  individual  in 
difficulties  and  risks  which  demand  either  intel- 
ligence or  else  so  great  profusion  that  dangers 
may  be  ignored.  Usually  it  is  the  young  who  are 
most  active.  Occasionally  it  is  the  reproductive 
adult,  as  in  insects.  Sometimes  the  power  of 
locomotion  is  confined  to  the  seeds  alone,  as  in 
many  plants.  It  may  be  reduced  to  a  creep  or 
crawl,  sometimes  it  is  a  parasitic  or  otherwise 
assisted  habit,  but  if  the  power  of  locomotion  did 
not  exist  the  species  could  not  spread.  The 
power  has  in  many  cases  developed  far  beyond 
obvious  necessity,  and  has  become  a  source  of 
joy,  as  in  the  bird,  or  a  great  interest  in  life,  as  in 
man. 

Breathing  and  feeding,  again,  including  the 
capture  of  food,  are  essential  functions  which 
have  evolved  organs  of  great  variety  and  interest. 
The  colours  and  the  senses  of  animals  are  closely 
connected  either  with  capture  or  with  escaping 
capture.  The  association  in  flocks  for  mutual 


INTRODUCTION  xi 

help  and  support,  the  care  of  the  young,  and  the 
deeply  interesting  laws  of  Heredity  and  Varia- 
tion, are  facts  which  run  more  or  less  through  the 
whole  realm  of  life,  nor  are  they  limited  to  the 
animal  kingdom  alone.  All  these  topics  are 
dealt  with  by  Professor  Gamble,  they  are  all 
topics  which  are  very  much  alive  at  the  present 
day,  and  the  information  that  he  gives  is  the 
kind  of  information  which,  in  a  general  way,  every 
educated  person  would  wish  to  possess. 

That,  I  take  it,  is  partly  the  object  of  the  series 
of  books  of  which  this  forms  one,  and  I  am  glad 
to  have  the  opportunity  of  commending  the 
series  to  that  increasing  number  of  readers 
who  are  hungry  for  trustworthy  and  assimilable 
information. 

One  of  the  most  striking  phenomena  which 
have  recently  come  to  light  is  the  deadly  effect 
of  too  long  continued  isolation.  Cells  can  multi- 
ply by  fission  or  mere  subdivision  for  a  certain 
period,  but  there  comes  a  time  when  unless  they 
are  rejuvenated  by  union  with  another  cell,  their 
power  becomes  feeble  and  the  whole  colony  decays 
and  dies.  Contact  with  another  or  supplemental 
cell,  sometimes  resulting  in  actual  combination, 
renews  the  vigour  of  both,  and  the  process  of 
fission  can  afterwards  go  on  for  much  longer. 
The  light  which  this  throws  on  the  origin  and 
deep-seated  necessity  of  sex  is  manifest;  and  the 
process  of  combination  or  fusion  of  individuals 


xii  INTRODUCTION 

who  had  developed  separately  is  curiously  remi- 
niscent of  the  inverse  process  imagined  by  Plato 
in  the  Symposium  as  an  historic  account  of  this 
strange  longing.  And  probably  Plato  intended 
something  quite  serious  by  this  legend;  though, 
as  if  to  safeguard  himself  from  too  literal  an 
interpretation  of  his  parable,  he  puts  it  into  the 
mouth  of  Aristophanes  in  order  that  it  might  be 
treated  as  a  joke  by  those  who  had  not  ears  to 
hear;  thus  safeguarding  his  position  somewhat 
after  the  same  fashion  as  Virgil  safeguarded  his 
vision  of  the  underworld — his  doctrine  of  pre- 
existence  and  reincarnation — from  the  scoffer, 
by  allowing  ./Eneas  and  the  Sibyl  to  issue  into 
upper  air  again  through  the  ivory  gate  instead  of 
through  the  gate  of  horn. 

It  is  through  the  gate  of  horn,  however,  that 
biologists  lead  us;  and,  in  the  light  of  our  extend- 
ing and  growing  knowledge  of  Physics  and  Chem- 
istry at  the  present  day,  there  can  hardly  be  a 
more  fascinating  study  than  that  of  the  fundamen- 
tal elements  of  life  and  the  action  of  material 
surroundings — whether  naturally  or  artificially 
supplied — on  the  life-history  and  development 
of  the  simpler  and  more  easily  studied  modes  in 
which  Life,  whatever  may  be  its  intrinsic  nature, 
incarnates  itself  upon  the  planet. 

OLIVER  LODGE. 


THE  ANIMAL  WOKLD 


CHAPTER    I 

THE   STRUCTURE   AND   CLASSIFICATION 
OF    ANIMALS 

IN  order  to  study  and  understand  animal  life 
it  is  necessary  to  have  a  few  clearly  defined  con- 
ceptions of  the  nature  and  range  of  animal  struc- 
ture and  of  the  genealogical  trees  on  which  animal 
pedigrees  are  founded.  To  many  people  this 
part  of  the  subject  is  less  interesting  than  the 
study  of  habits  and  of  life-histories,  but  without 
a  training  in  comparative  structure  and  classifica- 
tion, observations  and  experiments  are  of  little 
help.  We  must  know  what  is  the  material  we 
are  dealing  with  before  we  can  understand  its 
amazingly  varied  activities  and  adaptations. 
We  have,  in  fact,  to  detach  the  animal  from  the 
environment  before  we  can  appreciate  its  life. 
But  to  appreciate  animal  life  fully  we  have  to 
investigate  both  its  activities  and  its  structure. 

At  the  base  of  the  animal  kingdom  is  the  large 
and  important  division,  the  Protozoa.  The  Pro- 
13 


J4  THE  ANIMAL  WORLD 

tozoa,  or  "Animalcules"  of  older  writers,  are 
creatures  of  the  simplest  type  known,  with  the 
exception  of  the  bacteria,  about  the  nature  of 
which  there  is  some  doubt.  Excluding  microbes, 
however,  from  the  scope  of  this  book,  Protozoa 
take  the  lowest  place.  They  are  not  to  be  con- 
sidered as  primordial  in  the  sense  of  being  the 
first  created  animals;  as  to  that  we  have  no  evi- 
dence; but  they  undoubtedly  furnish  us  with  the 
simplest  self-contained  unit  of  animal  structure, 
and  their  remains  have  been  found  in  the  records 
of  the  rocks  as  far  back  as  any  indication  of  life. 
Indeed  the  flinty  or  limy  skeletons  of  these  animals 
help  to  build  up  the  rocks.  Many  sands,  lime- 
stones and  sandstones  of  certain  mountain  ranges 
are  largely  made  up  of  their  remains,  whilst  the 
pyramids  of  Egypt  are  vast  piles  of  a  Protozoon 
about  the  size  of  one's  finger  nail.  Protozoa 
abound  in  the  sea  and  in  fresh  water  in  all  lati- 
tudes. They  occur  in  soil  to  an  unknown  extent 
and  in  uninvestigated  variety;  and  they  occasion 
some  of  the  most  serious  diseases  from  which 
man  and  beast  can  suffer.  Sleeping-sickness  and 
malaria  are  two  of  the  most  notorious.  On  these 
several  accounts  the  study  of  Protozoa  has  been 
greatly  pursued  of  late  years. 

In  order  to  appreciate  the  structure  of  these 
animals,  we  must  refer  for  a  moment  to  the  organ- 
ized nature  of  higher  animal  tissues.  These 


STRUCTURE  AND   CLASSIFICATION   15 

tissues,  be  they  skin,  muscle,  nerve  or  bone,  are 
composite  structures,  and  the  component,  mic- 
roscopically small  units  out  of  which  they  are 
formed  are  termed  "cells,"  the  cells  of  each 
tissue  being  bound  together  by  a  varying  amount 
of  structureless  material  in  which  a  colourless 
fluid  circulates.  A  Protozoon,  however,  is  not 
compact  of  varied  tissues.  It  is  comparable  to 
a  single  cell,  to  one  of  the  multitude  that  make 
a  nerve-centre  or  that  crowd  a  blood-vessel.  It 
is  isolated  and  not  a  member  of  cell-society.  It 
lives  in  contact  with  the  world  and  is  not  shielded 
from  change  by  the  fluid  that  circulates  around 
a  tissue-cell.  It  must  feed,  move,  and  work  out 
its  being  by  itself  instead  of  being  bathed  with  a 
special  pap,  stirred  to  a  conjoined  exercise  and 
sheltered  from  contact  with  the  world  as  is  the 
case  with  a  tissue-cell.  The  latter  is  like  a  mem- 
ber of  a  community  living  his  or  her  own  life, 
but  doing  one  thing,  performing  one  office  for 
the  body  politic  out  of  which  livelihood  is  gained. 
The  Protozoon  is  like  a  Robinson  Crusoe,  who 
has  to  save  his  life  by  his  own  efforts  without  any 
chance  of  Man  Friday  giving  him  aid. 

This  isolation  of  the  Protozoon,  while  exposing 
it  to  many  dangers,  has  the  advantage  of  giving 
room  for  showing  inborn  capacity  for  adaptation. 
A  tissue-cell,  like  a  factory  hand,  has  to  do  one 
office,  and  to  keep  itself  well  enough  to  do  that, 


16 


THE   ANIMAL   WORLD 


Fig.  1.  —  Group  of  simple  Protozoa  (highly  magnified). 

A.  Amoeba   (X  200):    the  dark  spot  is  the   "nucleus"   or 

governing  centre  of  the  organism,  the  clear  space  is  the 
"contractile  vacuole,"  by  which  effete  matters  and  car- 
bon dioxide  are  discharged. 

B.  Gromia:   one  of  the  common  shore  Foraminifera  (  X  15)  ,7 

showing  the  protoplasm  streaming  out  of  the  test  or 
shell  and  capturing  diatoms. 

C.—Globigerina:  a  pelagic  foraminiferan  ( X  20),  showing 
the  chambered  shell  and  the  spines  with  which  it  is 
covered.  The  shells  or  tests  of  this  animal  form  an 
important  part  of  the  deep-sea  deposits  into  which 
they  fall  upon  the  death  or  upon  the  sporulation  of  the 
animal. 

D.  Rotalina:  this  is  another  important  element  in  globi- 
gerina  ooze,  and  is  also  found  in  many  limestones  (  X  6). 


STRUCTURE  AND  CLASSIFICATION     17 

but  a  Protozoon  is  the  servant  of  none.  Its 
energies  are  devoted  to  self -maintenance  and  the 
acquisition  of  that  place  in  nature  (whether  in 
water,  on  land,  or  as  a  parasite  in  some  animal) 
in  which  its  inborn  faculties  may  find  their  scope. 
The  result  of  this  plasticity  is  that  some  remain 
simple,  inert,  shapeless  lumps  of  moving  jelly; 
others  according  to  their  kind  develop  temporary 
motile,  hairlike  tentacles  singly  or  in  series  by 
which  they  row  themselves  through  water  or  some 
internal  fluid  of  their  host;  others  evolve  one  of 
several  means  of  buoying  themselves  in  the  ocean 
or  in  fresh  water,  as  by  growing  radiating  pro- 
cesses and  secreting  bubbles  of  jelly  which  they 
fill  with  gas.  Some  coat  their  bodies  with  a 
protective  envelope  in  which  they  imbed  foreign 
matter  for  greater  comfort;  some  deposit  lime 
m  shells  of  a  thousand  different  forms  constant 
in  each  "species";  others  construct  needles  and 
shells  of  flint  in  a  variety  of  graceful  and  often 
complex  patterns.  The  sea  is  often  full  of  such 
exquisite  and  imperishable  coronals.  A  pro- 
tozoon  is  therefore  not  necessarily  simple.  It 
has  simple  tools — a  mere  speck  of  protoplasm 
with  a  firmer  but  minuter  speck  or  two  at  its 
centre.  Yet  with  these  it  may  evolve  imper- 
ishable workmanship  of  the  most  exquisite  form, 
or  it  may  remain  as  it  was  born,  a  shapeless, 
unprotected  and  changeable  being.  The  fate 


18  THE  ANIMAL  WORLD 

of  each  is  decided  at  birth  and  no  effort  or  change 
of  environment  can  alter  it.  The  tools  are  al- 
most the  same  for  each.  The  work  of  one  will 
last  longer  than  the  hills  it  serves  to  form,  while 
that  of  the  other  leaves  no  mark. 

This  individualistic  life  is,  however,  a  short 
one.  How  long  a  tissue-cell  may  live  we  scarcely 
know.  A  nerve-cell  may  last  a  lifetime.  It 
remains  single.  But  the  other  cells,  bone-cell, 
skin-cell,  and  so  on,  are  short-lived.  Some  perish 
and  leave  no  descendants,  but  the  majority  merge 
themselves  in  daughter-cells  after  a  few  hours  or 
days  of  individual  life.  They  do  not  die,  but 
divide  and  become  twain.  In  some  disorganized 
tissue-cells  so  great  is  this  tendency  for  fission 
that  their  daughters  and  succeeding  generations 
break  the  bonds  of  their  confining  walls  and  pene- 
trate into  surrounding  tissues,  giving  rise  to  some 
form  of  that  dread  malady,  cancer.  Now  among 
Protozoa  this  multiplying  tendency  is  equally 
general.  The  body,  cased  or  free,  divides  and 
becomes  merged  in  its  offspring,  and  these  may 
at  once  separate  and  set  about  their  welfare  or 
may  live  together  in  a  common  envelope  if  the 
conditions  are  favourable.  In  this  way  we  re- 
ceive a  first  hint  of  those  animal  colonies  that 
become  such  striking  features  in  the  corals,  the 
sea-mats  (Fig.  17)  and  the  sea-fir.  The  Proto- 
zoon,  barring  accidents,  would  seem  to  be  im- 


STRUCTURE  AND  CLASSIFICATION     19 

mortal.     It  loses  itself  in  order  to  find  life  in  its 
descendants.     A  single  monad  (a  minute  speck 


Fig.  2. — Flagellate  Protozoa  (highly  magnified). 

A,  B.  Springing  Monads  in  the  flagellated  and  amoeboid 
states  (  X  900).  These  monads  are  extremely  common 
in  decaying  material,  in  foul  water,  also  in  the  bodies 
of  earthworms  and  in  soil.  The  body  is  provided  with 
one  or  more  whip-like  processes,  by  which  springing 
movements  are  executed,  but  at  certain  times  the 
flagella  may  be  withdrawn  and  replaced  by  pseudopodia 
as  in  B. 

C.  A  flagellate  (Copromonas)  from  the  intestine  of  the  Frog 

to  show  the  structure  (  X  900) .  On  one  side  of  the 
flagellum  is  seen  the  mouth  leading  into  a  narrow  tube 
down  which  food  has  passed.  The  nucleus  is  also  shown 
(with  black  centre). 

D.  Euglena:    a  green  flagellate  (  X  300),  very  common  in 
fresh  water.    This  animal  is  provided  with  green  chro- 
matophores    (starch-formers),    and    behaves    in    many 
ways  like  a  plant.    The  nucleus  is  seen  about  the  middle 
of  the  body. 

with  one  or  more  elastic  motile  cilia)  will  in  this 
way  multiply  to  the  million  in  a  few  hours,  in  an 
infusion  of  hay  (Fig.  2). 


20  THE  ANIMAL  WORLD 

But  as  though  the  hunger  for  life  were  not 
enough,  there  is  still  another  mode  of  perpetua- 
tion that  is  practised  by  most  if  not  by  all  of 
these  microscopical  creatures.  Simple  division 
carried  beyond  a  certain  number  of  cleavages 
appears  to  be  ineffective.  The  process  though 
long  is  not  endless,  and  after  a  time  it  slows  down 
and  ceases.  Exactly  why  it  should  do  this  is 
not  very  clear,  but  it  would  seem  that  the  slack- 
ening is  not  due  to  deterioration  in  the  environ- 
ment so  much  as  to  some  constitutional  weaken- 
ing. Any  weakness  is,  owing  to  the  simple 
cleavage  of  one  into  many,  conveyed  to  the  de- 
scendants and  there  seems  to  be  no  rectifying 
property.  It  has  been  found  that  if  such  a 
strain  or  culture,  the  members  of  which  are  de- 
rived from  a  single  Protozoon  by  repeated  fission, 
is  isolated,  it  gradually  dwindles  and  dies.  If, 
however,  it  is  allowed  access  to  another  culture 
of  exactly  similar  appearance,  it  undergoes  a 
renewal  of  this  dividing  property,  and  after  an 
interval  once  more  populates  the  water. 

This  "rejuvenescence,"  as  it  is  called,  is  bound 
up  with  a  process  known  as  conjugation,  which 
occurs  when  access  is  afforded  between  the  two 
strains  or  cultures  of  a  Protozoon.  Sometimes, 
as  was  said,  the  two  may  be  exactly  alike  so  far 
as  our  present  tests  can  go.  Sometimes  there  is 
a  marked  difference  (Fig.  3,  c)  between  the  in- 


STRUCTURE  AND  CLASSIFICATION    21 

dividuals  that  conjugate*  a  difference  of  size, 
complexity  and  movement.  In  either  case  the 
"gametes,"  as  they  are  called,  fuse  temporarily 
or  permanently  and  an  interchange  of  their 
several  living  substances  takes  place.  In  the 
former  case  the  gametes  then  separate  and  enter 
on  a  fresh  period  of  growth  and  division;  in  the 
latter,  the  two  bodies  fuse  into  one,  and  from  this 
immolation  there  proceeds  a  new  generation  of 
descendants  by  repeated  fission. 

The  study  of  conjugation  has  revealed  a  com- 
plexity of  occurrences  that  were  formerly  un- 
suspected and  that  cannot  be  stated  in  such  a 
brief  sketch  as  this.  What  is  of  chief  importance 
is  the  fact  that  associated  with  this  study  is  the 
discovery  of  sex.  It  is  now  known  that  even  the 
Protozoa  are  at  some  period  of  their  life  male 
or  female;  and  that  what  "rejuvenates"  a  colony 
or  stock  exhausted  by  mere  duplication  is  the 
impulse  received  by  members  of  another  sex. 
Not  in  all  Protozoa,  nor  in  these  at  all  times,  is 
the  male  as  definite  and  as  distinct  from  the 
female  as  it  is  in  those  figured.  But  the  distinc- 
tion is  there,  and  the  longer  these  Protozoa  are 
studied  the  more  clearly  is  seen  this  cleavage  into, 
and  union  between,  the  sexes. 

This  fact  of  sex  at  once  invalidates  our  com- 
parison of  a  Protozoa  with  a  tissue-cell.  Up  to 
a  point  the  simile  holds.  The  animalcule  is  a 


22 


THE   ANIMAL   WORLD 


A.  C. 

Fig.  3. — Ciliated  Protozoa  (Infusoria). 

A.  Paramecium,  the  slipper-animalcule  ( X  225) .  This 
abundant  infusorian  occurs  in  fresh  and  salt  water,  and 
also  in  cultures  produced  by  steeping  hay  in  water. 
The  body  is  slipper-shaped,  the  cavity  of  the  slipper 
being  the  mouth-cavity.  This  cavity  leads  into  the 
body-substance  as  shown,  and  the  food  (which  consists 
of  bacteria)  circulates  clock- wise.  The  large  black 
oval  spot  is  the  body-nucleus,  the  minute  spot  close 
to  it  being  the  germ-nucleus.  In  speaking  of  Protozoa 
as  single  cells  or  units  of  living  matter,  it  is  important 
to  bear  in  mind  that  they  frequently,  if  not  always, 
contain  two  perfectly  different  nuclei.  The  star-shaped 
structure  is  the  contractile  vacuole. 


STRUCTURE  AND  CLASSIFICATION    23 

unit  as  the  cell  is  the  unit  of  life.  But  it  is  some- 
thing more.  It  is  not  merely  a  self-sufficing  and 
self-duplicating  unit.  In  the  course  of  one  of 
these  incarnations  the  initial  Protozoon  becomes 
a  male  cell  or  a  female  cell  incapable  of  further 
life  if  not  given  access  to  its  supplementing  mate, 
casting  out  much  substance  to  effect  and  con- 
summate that  access  and  rejoicing,  if  we  may  so 
say,  to  found  a  race.  A  Protozoon  is,  therefore, 
a  cycle  of  events  in  part  of  individual,  in  part  of 
racial  incidents.  Each  incidental  individual  is 
not  a  single  cell  but  a  double  cell,  a  twin  star,  and 
is  therefore  in  virtue  of  that  essential  duplicity 
incomparable  with  any  one  of  the  constellations 
of  cells  that  we  call  a  higher  organism.  The 
whole  mass  of  dividing  individuals  represents 
the  body  of  that  organism,  whilst  the  male  and 
female  individuals  of  this  conjugating  period 
represent  the  germ-cells  of  the  higher  organisms. 


B.  A  group  of  Vorticella  (  X  100).     These  Protozoa  abun- 

dantly found  on  weeds  and  perched  on  small  animals, 
occur  both  in  fresh  and  salt  water.  Each  consists  of  a 
body  and  stalk.  The  body  is  surrounded  by  a  zone 
of  cilia,  by  means  of  which  bacteria  are  inhaled  and 
passed  into  the  mouth  (M) .  The  nucleus  (body-nucleus) 
is  a  coiled  structure,  and  the  germ-nucleus  is  extremely 
small. 

C.  The   ordinary    stalked   form    of    Vorticella,    attached   to 

which  is  a  minute  free-swimming  form  produced  by 
repeated  sub-division  of  a  stalked  individual.  The 
first  behaves  as  a  female  (?),  the  second  as  a  male  (cf). 
In  conjugation  the  male  is  absorbed  into  the  body  of  the 
female. 


24  THE  ANIMAL  WORLD 

METAZOA. — In  higher  animals  ("Metazoa") 
the  body  is  no  longer  a  single  cell,  but  is  composed 
of  a  vast  colony  of  cells  comparable  to  all  the 
myriads  of  cells  that  arise  by  fission  from  a  single 
Protozoon,  and  the  comparison  is  made  closer  by 
the  fact  that  each  multicellular  animal  arises  by 
subdivision  of  a  single  cell,  the  ovum  or  egg-cell. 
Some  factors  not  present  in  Protozoa  constrain 
the  fission-products  of  this  ovum  and  make  them 
cohere.  The  presence  of  calcium  is  one  of  these 
factors;  others,  however,  are  present,  for  the 
body  is  not  a  mere  heap  of  similar  cells.  It  is 
composed  of  tissues;  it  is  given  a  definite  form; 
it  grows  by  degrees  to  a  definite  size;  it  may  pass 
through  a  variety  of  experiences  and  phases 
before  reaching  adolescence,  and  then  may  aban- 
don the  individualistic  tendencies  that  it  has  so 
far  followed,  in  order  to  adopt  means  for  produc- 
ing and  protecting  its  family. 

Unlike  those  of  many  plants,  the  form,  struc- 
ture and  life-histories  of  animals  are  determined 
largely  by  innate  factors  and  depend  to  a  far 
slighter  extent  upon  environment.  In  a  similar 
way  the  relationships  of  animals  and  their  places 
in  classification  are  determined  largely  by  hidden 
facts  of  structure  rather  than  by  the  more  famil- 
iar and  external  features.  'In  fact,  it  is  the  more 
remote  and  valueless  features  of  animal  struc- 
ture that  are  the  clearest  signs  of  affinity,  for  being 


STRUCTURE  AND  CLASSIFICATION    25 

of  little  use  they  have  persisted  unchanged  whilst 
more  functional  organs  have  become  adapted  to 
various  ends.  The  silent  letters  in  many  words, 
such  as  the  "b"  in  doubt,  the  "g"  in  reign;  the 
buttons  on  our  coat-sleeves;  the  ears  on  our 


Fig.  4. — Illustrating  the  mode  of  division  of  a  Protozoon 
(Paramecium) .  M,  M',  the  new  mouths;  A,  anterior  end 
of  one  daughter-cell  ;  A',  posterior  end  of  the  other. 
M  and  N  are  the  great  and  small  nuclei  or  governing 
centres  respectively.  (After  Hickson.) 

faces,  are  such  survivals,  which  form  valuable 
evidence  of  relationship  between  words  in  differ- 
ent languages,  costumes  in  different  centuries, 
and  animals  of  different  kinds,  exactly  in  pro- 
portion to  their  uselessness  for  any  purpose. 
In  order  to  appreciate  animals,  therefore,  it  is 


26  THE    ANIMAL   WORLD 

necessary  to  go  below  the  surface,  and  to  use 
language  which  may  be  unfamiliar  but  has  the 
inestimable  advantage  of  definiteness.  Terms 
such  as  reptile,  insect,  worm,  fish  are  used  in  the 
vaguest  manner  and  have  no  scientific  meaning 
unless  defined.  Nor  do  the  vernacular  words 
for  structure  help  us  in  defining  the  genealogy 
of  animals;  nerve,  flesh,  bone,  skin  are  capable 
of  many  meanings  and  of  none;  a  more  precise 
nomenclature  is  an  absolute  necessity. 

Animals  are  by  nature  flexible,  and  usually 
motile  beings  needing  a  supply  of  solid  food  (pp. 
72-74),  which  they  convert  into  a  fluid  form  and 
then  diffuse  through  the  tissues.  This  primary 
fact  is  responsible  for  their  fundamental,  hollow 
nature.  An  animal,  disguise  it  how  we  may,  is  a 
tube.  It  consists  of  at  least  two  layers :  an  outer 
protective  and  sensitive  coat  and  an  inner  di- 
gestive and  also  sensitive  one.  One  or  two 
kinds,  it  is  true,  consist  of  a  single  coat  of  cells, 
but  it  is  not  at  all  certain  whether  these  extremely 
rare  creatures  are  as  simple  as  they  appear. 
They  may  turn  out  to  be  simplified  from  some 
more  highly  organized  and  double-layered  family, 
which  has  become  arrested  at  a  juvenile  stage  of 
its  life-history.  In  all  other  cases  this  one-lay- 
ered stage  becomes  converted  into  a  double- 
layered  tube,  if  not  indeed  into  still  more  highly 
organized  structures.  This  two-layered  condi- 


STRUCTURE  AND  CLASSIFICATION    27 

tion  is  the  limit  for  a  few  fresh-water  and  a  vast 
number  of  marine  animals,  and  is  expressed  by 
the  term  Ccelenterate.  All  hydroid  zoophytes, 
jelly-fish,  corals,  sea-fans  and  sea-pens  are  Coel- 
enterates,  and  consist  essentially  of  a  tube  with 
two  coats:  an  outer  one,  the  ectoderm,  and  an 
inner  one,  the  endoderm.  The  two  coats  are 
not  loosely  fitting,  but  are  bound  to  each  other 
by  a  greater  or  lesser  amount  of  soft  cement. 
In  Zoophytes,  the  amount  is  very  small,  but  in 
jelly-fish  and  corals  the  soft  intermediate  jelly 
is  bulky  and  confers  the  thick,  soft  character 
upon  these  animals.  Into  it  both  ectoderm  and 
endoderm  bud  off  cells,  and  these  play  the  middle- 
man between  the  two  primary  layers  as  well  as 
having  special  functions  of  their  own.  In  this 
way  increase  in  bulk  is  accomplished  without 
loss  of  touch  with  the  vitalizing  and  nourishing 
water.  This  middle,  usually  soft  tissue,  is  known 
as  mesenchyme. 

CCELENTERATES. —  We  may  now  briefly  de- 
scribe the  modifications  of  structure  found  within 
this  great  phylum,  the  Coelenterates.  The  body 
consists  of  a  tube  closed  at  its  lower  end.  The 
open  upper  end  is  the  mouth,  which  is  usually 
surrounded  by  a  number  of  tentacles  or  hollow 
outgrowths  of  the  tubular  wall.  Currents  of 
water  play  up  and  down  the  tentacles  and  body 
cavity  or  "Ccelenteron,"  carrying  food  and  oxy- 


28  THE  ANIMAL  WORLD 

gen  to  the  mesencnyme  and  so  to  the  ectoderm. 
Around  the  mouth  specially  nervous  processes  of 
the  ectoderm-cells  serve  as  a  rudimentary  nervous 
system,  whilst  longitudinal  outgrowths  of  both 
ecto-  and  endoderm  act  as  muscles  for  elongating 
or  shortening  the  body  and  tentacles.  There 
are,  however,  no  blood-vessels,  no  kidneys  for 
removing  effete  matters,  and  no  distinct  muscles 
(Fig.  5). 

The  body  has  usually  and  to  a  high  degree  the 
power  of  budding  so  as  to  form  colonies.  The 
buds  may  resemble  the  parent  and  remain  in 
connection  with  its  tissues  by  strings  of  mesen- 
chyme,  or  they  may  take  a  form  very  different 
from  the  parent-stock  and  separate  completely. 
Such  detached  buds  are  jelly-fish.  The  parent- 
stocks  of  practically  all  jelly-fish  are  fixed.  The 
free-buds  are  actively  pulsating  medusae  without 
any  trace  of  skeleton,  but  provided  with  an  en- 
hanced power  of  movement,  a  more  elaborate 
nervous  system,  and  with  organs  for  the  percep- 
tion of  light  and  the  maintenance  of  balance. 
Many  Coelenterates,  however,  have  no  free 
medusae.  The  sea-anemones,  corals,  precious 
corals,  sea-fans  and  sea-pens  have  only  fixed, 
not  free-swimming  polypes. 

These  flower-like  animals  are  incessantly  in- 
haling water  down  their  inverted  throats  and 
exhaling  it  through  one  or  two  special  grooves 


STRUCTURE  AND  CLASSIFICATION    29 


placed    at   one    or   both  angles  of  the  slit-like 
opening.     Their  length  of  life  is  considerable;  a 


Fig.  5. 


Fig.  6. 


Fig.  5. — Hydra,  the  common  fresh-water  polyp,  showing 
a  bud  on  one  side  and  an  ovary  (O)  and  testis  (T)  on 
the  other.  The  figure  is  taken  from  a  longitudinal 
section  through  the  body  in  order  to  show  the  simple 
structure.  The  cavity  (C)  opens  to  the  exterior  only 
through  the  mouth  (M),  and  has  a  double  wall,  the 
inner  layer  being  endoderm  (inner  skin),  and  the  outer 
one,  ectoderm. 

Hydra  is  abundant  in  fresh  water  all  over  the  world. 
(X8). 

Fig.  6. — Bougainvillea  ramosa,  a  marine  hydroid,  budding 
off  medusae.  (X  10.)  (After  Allmann.) 

sea-anemone    lives  for  fifty   years,  a  coral  for 
twenty-five.     From  time  to  time  they  emit  with 


30  THE  ANIMAL  WORLD 

the  exhalent  current  a  white  cloud  of  fine  particles 
which  are  the  male  germ-cells  formed  from  the 
endoderm,  and  these  are  carried  by  the  inhalent 
current  of  an  adjoining  individual  into  its  interior. 
If  they  meet  with  ripe  egg-cells,  fertilization 
follows,  and  in  a  day  or  two  a  cloud  of  larger 
though  still  minute  corpuscles  emerges  from  the 
female  stock.  These  corpuscles  are  the  "larvae" 
which,  after  drifting  a  while  through  the  waters, 
settle  down  and  form  new  corals. 

In  hydroids  the  germ-cells  or  gametes  are 
developed,  as  a  rule,  not  in  the  parent-stock  in 
which  they  arise,  but  in  the  medusa-buds  into 
which  they  wander.  Hence  when  a  medusa 
swims  away,  it  carries  with  it  a  stock  of  cells 
destined  to  be  cast  on  the  waters.  After  the 
eggs  are  fertilized  they  sink  to  the  bottom  and 
develop  into  hydroids.  The  medusa,  for  all  its 
organization,  is  but  a  sower,  a  disseminator. 

ACCELOMATA,     PLANARIANS,     ROTIFERS. The 

next  stage  of  animal  organization  is  effected  by 
the  growing  independence  of  the  tissue  we  have 
called  mesenchyme. 

In  Ccelenterates  the  mesenchyme  is  merely 
a  collection  of  cells  detached  from  both  the  inner 
and  outer  layer  of  the  body  and  performing 
subordinate  functions.  In  the  next  stage  of 
evolution  these  cells  form  an  independent  tissue, 
not  arising  directly  from  the  body-wall  but 


STRUCTURE  AND  CLASSIFICATION    31 

originating  early  in  life.  In  order  to  accommo- 
date this  tissue,  the  space  between  the  inner  or 
gut-wall  and  the  outer  wall  or  integument  is 
increased.  The  result  is  an  organism  which  has 
sufficient  elasticity  to  fit  it  for  creeping.  With 
this  deep-seated  change  of  structure  there  goes 
a  remarkable  change  of  habit  and  symmetry; 
in  place  of  the  old  fixed  or  floating  habit  and  ra- 
dial symmetry  there  is  now  found  a  freely  moving 
organism  with  right  and  left  sides,  and  a  definite 
ventral  surface  on  which  these  animals  creep: 
a  "head-end"  with  eyes  and  brain,  and  an  upper 
surface  variously  coloured  and  modified.  These 
are  the  first  creeping  things,  and  include  a  wide 
range  of  forms  for  which  no  convenient  name  is 
available.  Flat-worms;  Nemertines,  or  sea- 
worms;  Rotifers,  or  wheel-animalcules,  are  the 
most  useful  for  the  three  chief  divisions.  The 
characteristic  mark  of  these  animals  is  their 
unsegmented  triple-layered  body.  As  a  rule,  the 
mesenchyme  completely  fills  the  space  in  which 
it  lies,  and  its  cells  circulate  the  food,  perform 
the  creeping  movements,  expel  the  waste  nitro- 
genous products  of  life,  and  provide  the  germ- 
cells  by  which  the  race  is  maintained.  The 
outer  layer  or  ectoderm  is  usually  soft,  and  only 
in  Rotifers  does  it  build  up  an  investment  which 
protects  its  possessor  against  drought  and  attack. 
Its  most  important  property  is  to  furnish  the 


32  THE  ANIMAL  WORLD 

nervous  system,  which  consists  of  a  scattered 
mass  of  ganglia  lying  upon  the  ventral  body -wall, 
usually  concentrated  or  thickened  in  the  head- 
region  and  down  each  side  of  the  mid-ventral 
line.  It  is  interesting  to  note  that  the  power  of 
sustained  movement  in  a  definite  direction  which 
is  here  attained  for  the  first  time  in  our  survey, 
is  associated  with  the  higher  development  of  a 
nervous  system  and  of  sense-organs  by  which 
light  may  be  perceived. 

Flat-worms,  or  Planarians,  are  commonly 
found  in  the  sea  and  in  fresh  water,  creeping 
like  a  living  film  over  the  surface  of  stone  and 
shell;  in  tropical  and  warm  temperate  countries, 
land  planarians  occur  under  stones,  amongst 
earth  and  under  bark,  but  in  cold  temperate 
regions  they  are  very  rare.  As  is  the  case  with 
so  many  groups  of  animals,  that  of  the  Flat- 
worms  is  largely  addicted  to  parasitism.  Its 
free-living  members  are  carnivorous,  and  it  is 
but  a  short  step  from  the  habit  of  eating  flesh 
to  the  adoption  of  a  living  host  for  shelter  and 
food.  This  step  has  been  taken  by  the  "flukes" 
and  tape- worms.  In  its  more  extreme  forms  the 
influence  of  parasitism  is  profound  and  leads  to 
far-reaching  modification  of  structure  and  of 
life-history:  the  structure  of  the  individual  being 
sacrificed  in  order  to  ensure  the  continuation  of 
the  race. 


STRUCTURE  AND  CLASSIFICATION    33 

THE  CCELOMATA. — All  other  animals  are  char- 
acterized by  a  hidden  common  bond,  however 
diverse  their  appearance  and  general  anatomy 
may  be.  We  have  spoken  of  the  middle  tissue 
or  mesenchyme  which  plays  sustentative,  nutri- 
tive and  motor  parts  in  the  economy  of  the  Cce- 
lenterates  and  Flat-worms.  In  all  the  higher 
animals  (with  the  exception  of  one  or  two  prob- 
ably degenerate  groups)  there  is  formed  in  addi- 
tion to  the  mesenchyme,  a  definite  iollow  middle 
tissue  which  dominates  the  form  and  structure 
of  the  body,  and  ultimately  replaces  some  of  the 
mesenchyme  by  a  more  efficient  system  of  de- 
puratory  and  reproductive  organs.  This  hollow 
tissue  is  the  "Ccelom."  It  may  exist  side  by 
side  with  the  mesenchyme  or  may  displace  it 
altogether.  In  its  more  primitive  form  the  ccelom 
is  a  hollow  pouch  which  grows  out  from  the  food- 
tube  at  an  early  period  of  life  and  gives  rise  to  the 
muscles,  kidneys  and  germ-glands.  Usually  it 
also  forms  a  space  between  the  body-wall  and 
the  alimentary  canal  by  which  the  nourishment 
of  the  body  is  effected.  The  influence  of  the 
coelom  is  most  clearly  seen  in  the  form  of  the 
body.  It  appears  to  decide  more  than  any  other 
tissue  the  simplicity  or  segmentation  of  external 
form.  In  the  annelids,  or  segmented  worms  (of 
which  the  earthworm  is  a  familiar  example),  the 
co2lom  is  chambered  and  each  chamber  has  its 


34  THE  ANIMAL  WORLD 

outlet  to  the  exterior.  In  the  Arthropods 
(crustacea,  insects,  millipedes,  spiders  and  mites) 
the  repetition  of  parts  or  segments  is  also  clearly 
seen,  and  is  due  to  a  large  extent  to  the  presence 
and  subdivision  of  this  middle  tissue.  In  the 
vertebrate  animals  the  coelom  and  its  products 
are  of  the  greatest  importance,  for  they  give  rise 
to  the  vertebrae  and  the  muscles,  and  in  so  doing 
mould  the  shape  of  the  fish,  amphibian,  reptile, 
bird  and  mammal.  The  simplicity  of  unseg- 
mented  animals  is  also  an  outcome  of  the  un- 
divided nature  of  this  organ.  The  lamp-shells, 
the  Polyzoa  (such  as  the  sea-mat)  and  the  vast 
phylum  of  Mollusca  are  such  unsegmented  crea- 
tures, all  of  which  owe  their  want  of  subdivision 
to  their  compact,  undivided  coelom.  Perhaps 
the  clearest  example  of  the  moulding  influence 
of  this  somewhat  mysterious  structure  is  to  be 
seen  in  the  Echinoderms  (the  star-fish,  sea-urchins 
and  sea-lilies).  These  animals  have  a  radiate 
structure  as  symmetrical  as  that  of  a  coral.  They 
have  no  head,  no  brain,  and  consist  of  arms 
united  to  a  disc.  This  star-like  symmetry  is 
entirely  due  to  the  activity  of  the  coelom.  In 
early  life  a  star-fish  has  no  stellate  appearance. 
It  is  a  right-  and  left-sided  creature,  a  bilaterally 
symmetrical  one,  usually  bearing  elegant  ten- 
tacles by  which  it  swims  through  the  water.  Be- 
fore it  is  many  days  old,  however,  a  ccelomic 


STRUCTURE  AND  CLASSIFICATION    35 

pouch  grows  out  of  its  throat,  and  after  subdivid- 
ing, proceeds  to  form  a  small  radiate,  five-lobed 
mass  on  the  left  side  of  the  food-tube.  This 
stellate  internal  body  is  the  beginning  of  the 
future  star-fish,  the  entire  development  of  which 
is  governed  by  the  form  of  this  coelom  (Fig.  36, 
p.  250). 

SUMMARY. — From  what  has  been  said  in  the 
foregoing  paragraphs,  it  is  clear  that  the  classi- 
fication and  structure  of  animals  depends  largely 
upon  hidden  and  unfamiliar  factors,  and  that  the 
external  and  easily  accessible  characters  of  ani- 
mals are  the  result  of  profound  changes.  We 
may  look  upon  the  first  animals  or  Protozoa  as 
single  cells,  the  division  of  which  gives  rise  either 
to  similar  cells  or  to  "gametes"  which  are  usually 
either  male  or  female.  Higher  animals  keep 
their  cells  together  in  one  body,  which  in  the 
simplest  form  is  a  tube  opening  at  one  end  and 
composed  of  two  layers,  the  ectoderm  for  pro- 
tection, sensation  and  prehension,  the  endoderm 
for  digestion.  The  germ-cells  are  usually  re- 
tained in  the  parent-body,  but  may  be  discharged 
in  a  sort  of  raft  or  medusa.  Between  these  two 
layers  is  a  jelly;  and  the  evolution  of  this  jelly 
and  its  cells  is  one  of  the  chief  factors  leading  to 
the  development  of  higher  animal  forms.  In  the 
Coelenterates  the  jelly  and  its  contained  cells 
has  no  independence.  It  arises  from  the  two 


36 


THE   ANIMAL   WORLD 


SM. 


B 


Fig.  7. — To  show  the  ccelom  and  organs  derived  from  it. 

A.  A  diagram  of  a  sea-anemone  cut  longitudinally.  The 
mouth-tube  (M),  or  gullet,  is  drawn  out  at  its  angles 
into  two  grooves  or  sulci  (S).  These  grooves  create  an 


STRUCTURE  AND   CLASSIFICATION    37 

primary  layers;  but  in  the  Flat-worms  it  arises 
so  early  in  life  as  to  attain  independence  long 
before  birth.  This  middle  layer  is  called  mes- 
enchyme,  and  the  reason  for  its  development 
lies  in  the  need  for  active  movement.  This 
implies  better  circulation  of  food  and  removal  of 
waste  matters,  all  of  which  are  in  turn  the  out- 
come of  freer  life  and  sustained  locomotion.  In 
their  way  these  animals,  the  unsegmented  worms, 
have  learned  to  walk.  Their  brain  and  sense- 
organs  are  developed,  and  the  foundations  of  the 
conquest  of  the  earth  and  air  are  laid.  At  this 
stage  these  animals  consist  of  three  layers:  a 
sensitive  outer  layer,  a  digestive,  inner  one,  and 
between  these  a  sustaining,  contracting,  nourish- 
ing and  cleansing  tissue — the  mesenchyme.  But 
just  as  this  mesenchyme  has  supplanted  a  less 

outward-going  current  by  reason  of  their  cilia.  The 
gut  proper  is  pleated,  the  pleats  themselves  (SM)  being 
shown  on  edge  by  the  wavy  lines  F,  and  the  (black) 
spaces  between  the  pleats  form  the  ccelom. 

The  thickened  edges  of  the  pleats  are  digestive  fila- 
ments, arid  the  pleats  are  the  "septa."  The  digestive 
cavity  is  enormously  increased  by  this  outfolding. 

At  their  lower  ends  the  filaments  bear  ova  or  sperm- 
cells. 

B.  Diagram  showing  the  supposed  mode  of  transition  from 
A  to  Fig.  8.  This  diagram  is  based  upon  the  develop- 
ment of  primitive  Ccelomate  animals  (e.  g.  Peripatus 
and  Echinoderms) ,  which  show  that  the  primitive 
mouth  of  the  early  larva  becomes  converted  into  the 
two  openings  of  the  food-tube;  and  that  the  pi  eatings 
or  folds  of  the  primitive  gut  become  separated  off  early 
in  life  to  form  the  ccelom  (P). 

The  nervous  system  is  now  rising  up  as  a  fold,  en- 
closing both  ends  of  the  food-tube. 


38  THE  ANIMAL  WORLD 

efficient  jelly,  so  it,  in  turn,  is  replaced  more  or 
less  completely  by  another  dependency  of  the 
two  great  primitive  layers,  and  in  all  remaining 
animals  the  tissues  that  lie  between  the  skin  and 
the  alimentary  canal  are  compounded  largely 
out  of  this  new  "Ccelom,"  the  blood-vessels 
being,  perhaps,  the  only  exception.  Its  advent 
signalizes  a  further  advance,  and  the  form  it 
assumes  determines  that  of  the  body.  If  the 
ccelom  is  undivided,  so  is  the  body:  if  it  is  seg- 
mented, the  body  becomes  worm-like,  caterpillar- 
like,  fish-like:  if  it  is  lobed,  the  body  becomes 
stellate.  In  this  way  we  get  the  unsegmented 
Mollusca,  Polyzoa,  and  Brachiopods;  the  seg- 
mented worms,  Arthropods  and  Vertebrates; 
and  the  radiate  Echinoderms.  The  following 
table  summarizes  this  chapter: — 

Classification  of  Animals. 

1.  Protozoa:  Animals  that  remain  solitary  cells : 
each  composed  of  a  cell  that  divides  for  a  time 
and  then  requires  conjugation,  forming  a  zygote 
to  enable  it  to  divide  further.     Its  life  is  there- 
fore alternately  somatic  and  gametic. 

2.  Metazoa:  Animals   derived   from   a   zygote 
which  divides.     The  products  of  division  cohere, 
forming  a  tube. 

A.  The  tube  consists  essentially  of  two  layers 


STRUCTURE  AND  CLASSIFICATION    39 

enclosing  a  continuous  cavity.  The  walls 
are  folded  and  perforated  by  many  inhalent 
apertures  and  by  one  or  more  exhalent  open- 
ings. A  mesenchyme  present  .  .  Sponges. 

B.  The  tube  is  composed  of  two  layers  sepa- 
rated by  a  structureless  jelly  only  containing 
mesenchyme   in   the   higher   forms.     Sting- 
ing-cells present.     A  single  aperture  for  in- 
haling and  exhaling  water     .    .     Cwlenterates. 
(Hydroid      Zoophytes,      Medusae,      Corals, 
Anemones.) 

C.  The  tube  is  composed  of  three  layers,  the 
middle  or  mesenchyme  layer  arising  early  in 
life.     Body  bilaterally  symmetrical.     Tube 
with  one  (Planarians)  or  two  apertures  (Ne- 
mertines,  Rotifers) Accelomata. 

D.  Body  composed  of  two  tubes,  the  space 
between  the  two  being  lined  by  an  organ 
that  gives  rise  to  the  heart-sac,  the  kidneys 
and    the    germ-cells.       This    organ    is    the 
Ccelom Ccelomata. 

SYNOPSIS    OF    CCELOMATA. 

1.  Body  segmented  and  produced  into  hollow 
but    not    jointed    appendages.      Ccelom   exten- 
sive     Annelids. 

(Seaworms,      Earthworms,      Waterworms, 
Leeches.) 

2.  Body  segmented  and  produced  into  hollow, 


40  THE  ANIMAL  WORLD 

usually  jointed  appendages.     Ccelom  reduced  to  a 
small  kidney-sac,  and  to  the  germ-cells  Arthropods. 

(Shrimps,    etc.,    Insects,    Millepedes,    etc., 

Spiders,  etc.) 

3.  Ccelom  divided  but  not  segmented.     Body 
unsegmented.     Appendages  not  repeated  in  serial 
order.     Ccelom  (except  in  Cuttlefish),  as  in  Crus- 
tacea, i.  e.  not  segmented Mollusca. 

(Bivalves,  Univalves,  Cuttlefish.) 

4.  Ccelom    lobed,    extensive.     Body    radially 
symmetrical,  not  segmented  .    .    .     Echinoderms. 

(Starfish,  Sea-urchins,  Sea-lilies.) 

5.  Ccelom  segmented.     Body  enclosing  three 
tubes:   (1)  a  spinal  tube,  (2)  a  digestive  tube,  and 
(3)  a  ccelomic  tube.     Between  (1)  and  (2)  a  rod 
(notochord),    the    forerunner    of    the    vertebral 
column;   sides  of  (2)  perforated  by  slits  to  form 
gills  (Fig.  8) Chordata. 

(Vertebrata.) 

Classification  of  Vertebrata. 

1.  Notochord  lost  during  development 

Ascidians. 
(Sea-squirts,  Fig.  32.) 

2.  Notochord  retained  or  replaced  by  verte- 
bral column — 

A.  Median   (middle  line)   fins  with  supports. 
Paired  fins  without  digits   .   .     Fishes. 


STRUCTURE   AND   CLASSIFICATION    41 


ifi    gSl| 

i  a  o    6,2  §^ 


3  o, 


B,~£oot  hS-g 


'il  g-a 


U 


03 


42  THE  ANIMAL  WORLD 

B.  Median     fin     without     supports.     Paired 

limbs  with  digits — 

«.  With  gills  at  least  in  early  life  Amphibia. 
/?.  Without  gills  at  any  period — 

a.  Skull   with   one  knob  for  support   to 
vertebrse. 

aa.  Without  feathers     .    .    .     Reptilia. 
bb.  With  feathers Birds. 

b.  Skull  with  two  knobs  for  support 

Mammals. 


CHAPTER  II 

MOVEMENTS,     SUCCESSION     AND     DISTRIBUTION 
OF     ANIMALS 

MOVEMENT  is  one  of  the  attributes  of  being. 
Though  not  generally  recognized  among  plants, 
it  is  nevertheless  a  property  of  plant-life,  though 
a  property  that  is  exerted  only  in  a  reticent  fash- 
ion. In  many  animals,  on  the  other  hand, 
movement  is  a  dominant  note,  and  we  often  in- 
stinctively use  it  as  a  means  of  distinguishing 
animals  from  their  surroundings  when  walking 
over  the  country  side.  Such  movements,  both 
in  amount  and  finish,  vary  greatly.  A  sponge 
exhibits  no  active  change  of  position.  A  polype 
or  sea-squirt  moves  its  tentacles  only  or  shrinks 
into  its  shell,  a  hibernating  hamster  sleeps  for 


MOVEMENTS    OF   ANIMALS         43 

half  the  year  and  a  scale  insect  never  moves. 
On  the  other  hand,  a  whale,  a  lark,  or  a  dragon- 
fly is  the  embodiment  of  abounding  energy  and 
a  hovering-fly  the  acme  of  poise.  The  principle 
of  reserve,  of  not  working  up  to  breaking  strain, 
is  well  seen  in  the  usually  slow  movements  of 
quadrupeds.  A  herd  of  deer  moves  slowly  until 
an  alarm  excites  them. 

The  chief  reason  for  movement  is  to  be  found 
in  the  need  for  seeking  food.  Plants,  living  as 
they  do,  on  air  and  water,  can  grow  and  multiply 
without  obvious  change  of  place.  Animals  are 
unable  to  make  a  synthesis  of  air  and  water. 
They  require  either  solid  or  liquid  organic  sub- 
stances, and  only  after  analyzing  these  are  they 
able  to  synthesize  the  products  of  analysis  into 
living  matter.  Hence  animals  must  either  collect 
organic  matter  or  go  seeking  it.  This  quest 
exposes  them  to  many  dangers:  the  fury  of  the 
elements,  the  attacks  of  enemies,  the  danger  of 
being  lost;  and  hence  movements  have  to  be 
adjusted  to  meet  a  variety  of  circumstances. 
The  food  of  a  young  animal  is  not  to  the  taste 
of  an  older  one.  Habits  change  and  movements 
change  along  with  them.  Food,  again,  is  not 
continually  required.  By  day,  in  some  cases,  by 
night  in  others,  there  ensues  a  period  of  repose 
when  the  cud  can  be  chewed  or  sleep  obtained. 
The  fall  of  the  tide  is  the  signal  to  thousands  of 


44  THE   ANIMAL  WORLD 

creatures  for  a  period  of  enforced  rest,  to  others 
of  renewed  activity.  Hard  winters  in  temperate 
or  arctic  lands  cast  many  animals  into  a  deep 
sleep.  The  dry  season  of  the  tropics  induces  a 
profound  stupor  in  many  others.  These  intervals 
of  rest  and  activity  induce  many  movements 
that  are  unconnected  with  food,  and  that  are 
directed  rather  to  the  acquisition  of  compara- 
tive safety.  The  frog  that  spends  the  summer  in 
lush  grass  seeks  the  bottom  of  a  pond  during 
winter.  The  earthworm  descends  the  soil  to  es- 
cape frost.  Caterpillars  leave  their  dying  food- 
plants  and  hibernate  in  moss  or  in  the  grcund. 
The  mud-fishes  of  South  America  and  of  Africa 
burrow  in  the  bank  at  the  approach  of  drought, 
and  lie  dormant  until  the  next  rains.  The  frogs 
of  Australia  dig  holes  with  their  ankles  to  escape 
the  dry  season.  The  Annelids  and  Crustacea 
of  our  coasts  retire  at  the  fall  of  tide  into  their 
burrows  to  escape  desiccation.  Movement,  there- 
fore, is  directed  not  only  towards  sources  of  food 
but  to  self -protection.  It  is  not  a  continuous 
phenomenon,  but  one  subject  to  periodic  stop- 
pages. The  time  of  the  day,  the  season  of  the 
year,  the  phase  of  the  moon,  the  temperature 
of  the  ground,  influence  movement  profoundly: 
now  enhancing,  now  inhibiting  it,  or  directing  it 
into  new  channels,  in  relation  to  the  welfare  of 
the  individual. 


MOVEMENTS  OF  ANIMALS        45 

In  addition  to  the  movements  that  conduce 
to  individual  welfare  there  is  another  class  of 
motions  that  relates  to  racial  well-being.  Many 
animals,  such  as  barnacles,  hydroids,  anemones, 
corals,  are  fixed,  and  only  perform  small  vibra- 
tions within  a  short  radius  of  their  station.  Some, 
such  as  the  snails,  bivalves  and  other  unseg- 
mented  creatures,  though  not  fixed,  are  seden- 
tary and  cover  little  ground  in  a  lifetime.  All 
these  fixed  animals,  and  the  majority  of  the  slug- 
gish ones,  have  an  active  young  stage.  They 
hatch  usually  as  free-swimming  larvae,  the  organ- 
ization of  which  is  usually  very  different  from 
that  of  their  parents  (Chapter  VIII).  This 
active  juvenile  stage  needs  sense  organs,  diges- 
tive organs  and  perceptions  adapted  to  a  life 
more  full  of  incident  than  that  of  their  inert 
parents. 

The  object  of  this  more  moving  period  is  not 
so  much  the  quest  of  food  as  the  colonizing  of 
new  regions.  If  the  young  were  lethargic,  the 
habitat  of  parents  and  children  would  soon  be 
depleted  of  its  food  reserves.  It  is  to  the  advan- 
tage of  both  and  of  the  race  that  new  regions 
should  be  explored  and  new  colonies  started  by 
the  explorers,  and  incidentally  other  advantages 
are  secured.  The  tendency  of  sluggish  life  is 
toward  the  depreciation  of  delicate  structure,  and 
though  the  conservative  power  of  structure  is, 


46  THE  ANIMAL  WORLD 

undoubtedly,  great,  yet  in  the  course  of  genera- 
tions, loss  of  eyes,  of  muscles,  even  of  the  head 
itself  may  occur  as  the  result  of  a  sluggish  tem- 
perament and  unchanged  conditions. 

Against  this  tendency  the  active  young  stage 
of  animals  is  some  preventive.  It  ensures  that 
at  least  once,  or  for  one  period,  the  body  shall  be 
alert,  the  senses  quick,  the  muscles  taut.  It 
involves  a  change  to  a  new  environment  where 
the  flow  and  ebb  of  tide,  the  denser  and  lighter 
water,  the  changes  of  surroundings  are  more 
marked.  It  ensures  that  the  most  adaptable 
of  the  family  thus  launched  upon  unfamiliar 
ways  shall  in  at  least  many  cases  be  the  ones  to 
survive  and  in  new  regions  to  maintain  the  race. 
The  practice  of  movement  by  young  animals  not 
only  fits  them  for  their  life,  but  keeps  up  the  vir- 
ility of  the  race.  Animals  do  not  play  because 
they  are  young.  They  are  young  in  order  that 
they  may  play. 

Animal  life  is,  however,  strangely  varied,  and 
its  youth  is  often  an  inert  phase  rather  than  an 
active  one.  Where  food  is  plentiful  and  easily 
acquired  it  is  often  found  that  the  benefit  of  the 
race  is  fostered  rather  by  sluggishness  on  the  part 
of  its  youth  and  by  activity  on  that  of  its  age. 
Growth  in  all  animals  is  rapid  at  first,  slow  after- 
wards: and  there  are  many  orders  that  devote 
their  youth  or  larval  stage  to  rapid  growth  fa- 


MOVEMENTS  OF  ANIMALS        47 

voured  by  abundant  food  and  little  exercise.  In- 
sects form  an  admirable  example  of  this  adaptive 
growth.  In  the  majority  of  insect  orders,  the 
parents  are  eminently  active,  the  young  eminently 
passive.  In  them  growth  and  maturity  have  to 
be  rapidly  gained  in  order  that  brood  after  brood 
may  possess  the  summer.  Thus  it  comes  about 
that  movement  is  adapted  to  racial  ends.  In 
some  animals  and  in  some  phases  of  growth  it 
may  be  reduced  or  even  lost;  but  what  is  lost  in 
one  stage  is  regained  in  another.  The  funda- 
mental property,  movement,  is  never  wholly 
absent  from  the  cycle. 

MOVEMENTS  OF  PROTOZOA. — The  oldest  forms 
of  movement  are  those  performed  by  Protozoa; 
and  consist  of  the  extrusion  of  any  part  of  the 
mobile  body  or  the  contraction  of  some  especially 
motor  process  of  it.  In  the  former  case  the  direc- 
tion of  movement  is  indeterminate,  its  rate  slow 
and  its  relation  to  the  substratum  assured.  In 
the  latter,  the  Protozoa  is  carried  rapidly  through 
the  water  in  a  definite  direction  and  is  rendered 
independent  of  the  soil.  Such  microscopic  organs 
as  serve  to  this  end  are  like  minute  eyelashes  and 
hence  are  called  "cilia."  In  higher  animals 
they  are  used  either  for  the  same  purpose  as  in 
Protozoa,  namely,  to  seek  food  or,  if  not  strong 
enough  for  this,  to  inhale  food  from  the  sur- 
rounding water,  and  to  expel  from  the  body 


48  THE  ANIMAL  WORLD 

noxious  substances  that  tend  to  accumulate 
within  it. 

CCELENTERATES. — In  Coelenterates,  cilia  are 
still  the  chief  motor  organs,  but  in  this  phylum 
we  find  a  primitive  kind  of  muscular  tissue  used 
for  retracting  the  mobile  part  of  the  body  and 
in  jelly-fish  for  swimming.  The  cilia  are  of  two 
kinds:  those  which  create  an  inwardly  directed 
current  into  the  mouth  and  those  which  are  con- 
stantly exhaling  water  through  the  throat-tube 
so  as  to  keep  up  the  circulation.  In  general,  how- 
ever, the  Ccelenterates  are  sessile  and  not  freely 
moving  organisms,  and  in  consonance  with  their 
sluggish  habit  the  nervous  system  and  sense 
organs  remain  simple.  Only  in  the  Medusae  are 
they  highly  and  definitely  organized. 

MOVEMENTS  OF  THE  HIGHER  ANIMALS. — It 
is  not  until  we  reach  the  Accelomata  that  muscu- 
lar movement  becomes  the  motive  power  of  creep- 
ing. In  this  phylum  a  definite  brain  is  associated 
with  the  habit  of  creeping  with  one  end  of  the 
body  constantly  in  front.  But  even  in  these  ani- 
mals movement  is  only  fitful  and  slow.  The 
Planarian  glides  away  only  to  rest  as  soon  as  it 
reaches  a  shady  corner  or  when  day  breaks.  It 
trails  the  easily  lacerated  body  behind  it  and  has 
no  freedom  of  action,  and  we,  therefore,  do  not 
wonder  that  its  parasitic  relatives  have  so  soon 
given  up  the  habit  of  movement.  It  appears  as 


MOVEMENTS  OF  ANIMALS 


49 


though  mesenchyme  were  not  an  efficient  means 
of  producing  sound  muscular  tissue,  capable  of 
bearing  the  body's  weight  as  well  as  of  propelling 
it  rapidly.  The  advent  of  the  coelum  brings  the 
desired  change.  The  most  active  animals  are  in- 
sects and  vertebrates,  two  divisions  of  the  Ccelo- 
mata,  and  it  can  be  shown  that  the  muscles  of 


Fig.  9. —  The  flying  fish  (Exocoetus  volitans),  to  show  the 
pectoral  fins,  which  form  an  aeroplane  during  flight, 
and  the  small  pelvic  fins  (P)  corresponding  to  the  legs 
of  other  vertebrates. 

these  creatures  are  derived  from  the  segmented 
masses  which  lie  between  the  gut  and  the  body- 
wall  in  the  embryo;  in  other  words,  from  the 
coelum  (Fig.  8,  B).  Hardly  less  active  than  a  fish 
is  a  cuttlefish,  the  most  highly  organized  of  the 
Mollusca,  and  it  is  just  in  this  subdivision  of  the 
phylum  that  the  ccelom  attains  the  greatest  de- 
velopment. Thus  we  may  state  as  a  general  con- 
clusion, though  subject  to  certain  qualifications, 
that  the  power  of  sustained  rhythmic  muscular 


50  THE  ANIMAL  WORLD 

contraction  is  developed  in  the  Ccelomata  from 
cells  which  form  part  of  the  walls  of  the  ccelom. 
The  simplest  arrangement  of  the  muscles 
takes  the  form  of  a  double  layer  inserted  into 
the  skin,  the  outer  layer  being  circular  in  direc- 
tion and  the  inner  layer  longitudinal.  By  con- 
traction of  the  first  and  relaxation  of  the  second, 
the  body  is  elongated  and  made  thinner,  whilst 
it  thickens  and  shortens  by  the  reverse  action. 
The  muscles  may  be  segmented,  as,  for  instance, 
in  Arthropods,  Annelids  and  Vertebrates,  and  the 
circular  layer  may  be  dispensed  with  as  the  body 
becomes  more  rigid;  but  the  majority  of  aquatic 
animals  move  by  alternately  bending  their  right 
and  left  sides  into  a  sinuous  curve,  and  so  gaining 
a  fulcrum  at  the  hollows  against  the  medium  in 
which  they  live.  Thus,  fish  swim  by  lateral  un- 
dulations of  the  muscle-columns  and  of  the  fins 
that  run  down  the  centre  of  the  back  and  of  the 
belly,  and  are  assisted  by  a  torsion  of  the  tail- 
fin,  which  turns  through  a  figure  of  eight.  Very 
slow  movements  are,  it  is  true,  effected  by  back- 
ward strokes  of  the  paired  fins,  especially  of  the 
pectorals  (corresponding  to  our  arms),  but  in 
rapid  movement  these  fins  are  used  rather  for 
balancing  and  steering  than  for  propulsion;  and 
the  muscles  of  the  paired  fins  are  merely  local 
developments  of  the  segmented  body-muscula- 
ture and  therefore  are  ccelomic  in  origin.  Hence 


MOVEMENTS  OF  ANIMALS         51 

the  fins  move  altogether  if  they  move  at  all.  There 
is  no  system  of  joints  in  a  fish's  fin  such  as  we 
meet  with  in  the  limbs  of  higher  vertebrates, 
for  the  weight  of  the  animal  is  not  felt,  and  pro- 
pulsion, not  sustenance,  is  the  needful  mechanical 
action. 

When  we  come  to  land  animals,  and  still 
more  when  the  problem  of  flight  is  considered, 
the  problem  of  weight  has  to  be  considered  be- 
fore that  of  locomotion.  The  lateral  undulations 
of  the  body,  even  when  aided  by  unjointed  pad- 
dles or  fins,  are  not  sufficient  to  ensure  rapid 
movement  on  land.  Hence  a  system  of  levers  has 
to  be  evolved,  partly  to  support  the  body  and 
partly  to  propel  it.  The  use  of  joints  becomes  a 
necessity,  and  we  find  that  all  active  terrestrial 
animals,  except  snakes,  have  jointed  limbs.  In- 
sects, for  example,  have  three  pairs  of  jointed 
legs,  the  first  pair  for  haulage,  the  middle  pair  for 
balance  and  support,  and  the  last  pair  for  thrust- 
ing. Each  leg  becomes  converted  into  a  more 
efficient  limb  than  is  found  in  their  remote  con- 
nections, the  Millepedes,  or  their  still  more  dis- 
tant relations,  the  Crustacea.  In  these  creeping 
forms  the  legs  are  numerous,  the  joints  of  the  legs 
approximately  alike;  whereas  in  insects  the  body 
is  nicely  balanced  about  its  middle  or  thoracic 
part,  and  each  leg  is  made  up  chiefly  of  a  strong 
femur  or  thigh,  a  long  shank  or  tibia,  and  a 


52  THE  ANIMAL  WORLD 

jointed  foot  or  tarsus,  suitable  for  obtaining  a  firm 
and  yet  flexible  grasp.  The  body  of  an  insect  is, 
in  consequence  of  the  length  of  the  tibiae,  raised 
above  the  ground,  and  therefore  requires  constant 
adjustment  in  order  to  maintain  its  balance.  To 
meet  this  end  the  nervous  and  muscular  systems 
are  highly  organized.  This  example  is  sufficient  to 
show  that  the  general  result  of  terrestrial  adapta- 
tion is  an  advance  on  that  requisite  for  aquatic  life. 
Vertebrate  evolution  is,  however,  the  classical 
example  of  this  result.  The  critical  point  in  the 
history  of  this  phylum  is  passed  when  its  mem- 
bers migrated  from  water  to  the  land.  The  step 
was  taken  by  the  ancestors  of  the  Amphibia  (that 
is,  the  frogs,  toads  and  salamanders) .  In  them  the 
breast-fins  of  the  fish  have  become  the  jointed 
fore-legs,  the  pelvic  fins  have  become  the  hind-legs. 
As  in  the  insects,  the  limb  has  become  segmented 
into  three  portions,  a  thigh,  shank,  and  foot,  and 
the  number  of  webbed  rays  has  diminished  to 
five  or  four,  the  end  joints  of  which  become  free 
and  form  the  digits.  These  are  unable  to  perform 
the  movement  of  opposing  the  first  digit  to  the 
rest,  which  is  such  an  essential  factor  in  the  use- 
fulness of  our  own  hands,  and  the  wrists  and 
ankles  are  stiff.  How  this  great  change  from  the 
fish-fin  to  the  five-fingered  hand  occurred  is,  at 
present,  just  as  obscure  as  the  mode  of  conversion 
of  the  arms  of  reptiles  into  the  wings  of  birds. 


MOVEMENTS  OF  ANIMALS          53 

The  answer  can  only  be  supplied  by  further  dis- 
coveries in  the  geological  history  of  the  order,  and 
though  this  history  can  be  traced  back  to  the 
time  of  the  Coal  Measures,  we  find  the  earliest 
Amphibia  as  sharply  marked  off  from  the  fishes 
by  their  feet  as  they  are  to-day. 

The  earliest  Amphibia  give  rise  to  two  im- 
portant orders,  which  have  undergone  very 
different  lines  of  evolution.  In  one  direction  there 
were  evolved  (Fig.  10)  the  sluggish  newts,  sala- 
manders and  frogs,  which  still  scarcely  raise 
themselves  above  the  ground;  some,  indeed,  have 
degenerated  in  the  matter  of  their  limbs,  and  now 
live  in  equatorial  countries  like  earthworms  in 
the  soil,  having  lost  every  trace  of  their  appen- 
dages; others,  the  water-newts,  use  their  feet 
only  in  a  vague  and  tentative  way,  depending 
chiefly  upon  the  tail  and  lateral  body-muscles  for 
swimming  movements.  The  salamanders,  eman- 
cipated from  the  water,  are  confined  to  damp 
woods  or  the  borders  of  the  snow-line,  where  per- 
petual damp  can  be  secured,  and  these  move 
slowly  without  the  use  of  their  muscular  tail. 
The  frogs  and  toads  develop  the  tail  in  the  larval 
or  tadpole  stage,  when  they  still  swim  like  fish, 
but  absorb  it  for  the  building  up  of  the  powerful 
hind-limbs  when  the  period  of  metamorphosis 
ensues.  Though  capable  of  active  movement  and 
endowed  with  keen  senses,  these  Amphibia  have 


54  THE  ANIMAL  WORLD 

done  little  to  acquire  a  higher  place  in  nature  than 
their  forefathers  of  the  Coal  Measures. 


Fig.  10. — Group  of  Amphibia. 


A.  Male  of  crested  newt  Triton  (Molge)  cristatus,  showing 

the  high  dorsal  fin  and  caudal  fin  encircling  the  tail. 
The  segmented  muscles  are  seen  by  the  transverse 
lines  crossing  the  body.  (  x  1.) 

B.  Siren:    a  North  American  newt  which  has  lost  its  hind- 

limbs  and  retained  its  external  gills.     (  X  ?.) 

C.  Ichthyophis:     a   Central   American   amphibian   that   has 

lost  both  pairs  of  limbs.  It  belongs,  therefore,  to  the 
Apoda,  or  amphibia  which  live  in  earth  and  manure- 
heaps.  The  specimen  shows  a  female  guarding  her 
eggs.  (  X  f  •)  (All  after  Gadow,  Cambridge  Natural 
History.) 

EVOLUTION  OF  THE  REPTILIA. — To  this,  the 
second  group  offers  a  complete  contrast.     The 


MOVEMENTS  OF  ANIMALS         55 

reptiles  form,  indeed,  a  sister  order  to  the  Am- 
phibia, descended  as  they  are  from  an  early  stock 
of  newt-like  forms,  but  with  how  different  a 
career.  Their  emancipation  from  aquatic  life  was 
more  rapidly  completed,  their  locomotory  powers 
early  attained  a  degree  of  cultivation  that  no  Am- 
phibian has  ever  equalled.  By  its  darting  move- 
ment a  lizard  easily  evades  the  hand  stretched  out 
to  capture,  and  the  slow  and  quick  movements  of 
a  snake  have  an  almost  uncanny  elusiveness  and 
strength.  But  it  is  rather  in  the  extinct  groups 
than  in  the  modern  forms,  that  reptiles  show  their 
variety  and  grace  of  movement.  From  an  obscure 
beginning  (of  which  the  Hatteria,  or  rare  Nor- 
folk Island,  New  Zealand,  Tuatara  is  the  sole 
representative),  the  reptiles  flowered  out  into  a 
vegetarian  and  carnivorous  beast-like  race  that 
flourished  in  Africa,  Europe,  and  Asia.  In  these 
creatures  the  body  was  carried  clear  of  the  ground 
and  had  probably  the  movements  of  a  dog.  So 
close  indeed  is  the  similarity  of  many  of  the  bones 
of  this  early  group  to  those  of  mammals  that  these 
reptiles  are  termed  the  Theromorphs,  and  have 
been  thought  the  putative  parents  of  the  highest 
animals.  From  the  Theromorphs  onwards  the 
history  of  reptiles  is  full  of  incident.  Some  groups 
attained  the  mastery  of  the  land,  others  that  of 
the  sea,  and  one  group  at  least  learned  to  fly.  In 
consonance  with  these  deviations  of  habits,  modi- 


56  THE  ANIMAL  WORLD 

fications  of  structure  went  hand  in  hand.  Vast 
length  and  bulk  characterized  the  most  successful 
terrestrial  and  aquatic  orders,  but  did  not  pre- 
vent the  co-existence  of  many  small  races.  In 


Fig.  11.  —  One  of  the  extinct  flying  reptiles  in  the  position  of 
rest  (Pterodactylus  longirostris) ,  showing  the  immensely 
elongated  "little"  finger  which  bears  the  wing  mem- 
brane. (After  Seeley,  X  J.) 

the  Dinosaurs  there  are  giant  forms,  whose  re- 
mains are  to  be  seen  at  Brussels,  that  stand  on 
their  hind-limbs  and  could  reach  with  their  hands 
the  juicy  shoots  of  trees,  twenty  feet  high,  and 
there  are  delicate,  almost  bird-like  forms  not  big- 
ger than  a  gull.  In  these  Dinosaurs  the  bipedal 
habit,  now-a-days  only  retained  by  birds,  kanga- 


MOVEMENTS  OF  ANIMALS         57 

roos  and  man,  was  the  rule.  On  the  other  hand, 
the  flying  reptiles  or  Pterosauria  were  also  of  very 
different  kinds,  all  of  which  were  perfectly  dis- 
tinct from  any  animals  that  exist  to-day.  The 
"  little-finger  "  in  all,  is  enormously  elongated  and 
strengthened,  and  supports  a  wing-membrane 
upon  its  inner  edge.  Also,  unlike  birds,  both  the 
fore  and  the  hind-limbs  are  clawed,  and  may  be 
long  like  those  of  deer,  or  unequal  in  length  like 
those  of  bats.  In  size  these  animals  varied  from 
that  of  a  wren  to  vast  creatures  twenty  feet  in 
spread  of  wing.  No  less  varied  were  the  "  whales  " 
of  that  period.  Many  and  distinct  orders  returned 
to  the  element  from  which  ages  of  evolution  had 
removed  them,  and  here  they  preyed  on  their 
less  emancipated  associates,  the  fish,  and  acquired 
varied  forms.  Some,  with  elongated  necks  and 
paddle-like  limbs,  have  no  counterpart  in  other 
ages.  A  few,  the  Ichthyosauria,  had  short  necks 
and  long  heads  like  the  true  whales,  and  in  all 
these  aquatic  reptiles  the  tail  was  the  chief  swim- 
ming organ.  Such  mastery  of  the  earth,  air,  and 
sea  would  appear  to  exhaust  the  possibility  of 
evolution.  The  reptiles  seem  secure  for  ever 
against  competition.  So  little  do  we  see  of  the 
forces  that  displace  kingdoms  that  we  even  now 
know  nothing  of  the  causes  that  lead  to  their 
disappearance,  an  exit  as  impressive  as  anything 
in  nature.  Lizards,  tortoises,  crocodiles,  and 


58  THE  ANIMAL  WORLD 

snakes  alone  survive  to  maintain  the  traditions 
of  a  mightier  past. 

LOCOMOTION  IN  THE  MAMMALIA. — The  his- 
tory of  mammals  is  an  equally  moving  spectacle. 
Beginning  probably  in  the  time  of  early  reptiles, 
the  first  mammals  were  a  feeble  folk.  Their  ear- 
liest known  traces  occur  at  the  heydey  of  their 
saurian  relatives,  and  show  some  to  have  been 
small  opossum-like  beasts,  others  were  probably 
different  from  anything  that  now  exists,  but  it 
was  not  until  the  fall  of  reptiles  that  the  mam- 
malia assumed  their  variety  and  attained  their 
commanding  position.  Their  modes  of  progres- 
sion have  taken  in  the  main  two  forms:  increased 
running  and  leaping  powers,  due  to  elongation  of 
the  limbs,  shortening  of  the  tail  and  development 
of  large  extending  and  flexing  muscles;  and  a 
more  or  less  completely  bipedal  form  of  locomo- 
tion by  which  the  fore-limbs  are  set  free  for 
grasping,  and  in  consequence  of  which  the  manual 
arts  have  become  possible.  Such  partial  emanci- 
pation of  the  arms  for  the  support  of  the  body 
is  seen  in  the  kangaroos,  the  extinct  sloths,  the 
lemurs,  monkeys  and  apes.  In  man  alone  is  it 
complete,  and  has  led,  as  much  perhaps  as  any 
single  structural  feature,  to  his  evolution,  when 
we  consider  the  immense  effect  of  signs,  words, 
of  drawing  and  art  and  of  manual  work,  in 
bringing  about  civilization. 


MOVEMENTS  OP  ANIMALS         59 

Special  forms  of  locomotion  are  rare.  Bats 
are  the  only  true  fliers  in  the  order,  but  at  least 
three  other  orders  have  members  in  which  a 
large  supporting  membrane  stretched  between 
the  arm  and  the  body  enables  them  to  make 
flying  leaps.  The  flying  phalangers  of  Australia, 
the  flying  squirrels  of  Africa,  and  the  "flying 
Lemur"  or  Galeopithecus  of  Malaya  are  the  best 
known  examples.  The  whales  illustrate  another 
means  of  locomotion.  In  these  mammals  the 
hair  is  reduced  to  a  few  bristles  and  the  hind- 
limbs  have  disappeared;  the  body  terminates  in 
a  horizontally  placed  pair  of  flukes  made  up  of 
the  tendons  of  the  abdominal  muscles,  and  it  is 
by  the  action  of  these  muscles  that  the  stroke  is 
effected.  The  enormously  increased  length  and 
bulk  of  whales  over  those  of  their  terrestrial  allies 
is  an  interesting  commentary  on  the  need  for 
supporting  and  limiting  the  weight  of  the  body 
on  land.  There  is  no  doubt  that  whales  have 
been  descended  from  terrestrial  ancestors,  al- 
though, as  in  so  many  other  cases  of  profound 
change,  we  are  not  as  yet  able  to  point  with  cer- 
tainty to  the  ancestral  group.  But  there  is  also 
no  doubt  that  the  increase  in  bulk  occurred  after 
the  adaptation  of  aquatic  life.  The  weight  no 
longer  has  to  be  supported  by  muscle,  and  there 
is  no  such  limiting  factor  as  existed  before  in  the 
strength  of  the  limbs.  Hence  the  possibility  is 


60  THE  ANIMAL  WORLD 

given  for  greatly  increased  size.  The  factors, 
however,  that  determine  why  a  rorqual  can  at- 
tain seventy  feet  in  length,  whilst  the  porpoise 
never  exceeds  a  few  feet  are  still  largely  uninves- 
tigated. 

One  of  the  favourite  starting-points  for  the 
origin  of  whales  has  been  the  family  of  which 
seals  are  the  best  known  members,  and  these 
animals  afford  an  excellent  example  of  ter- 
restrial and  aquatic  life.  There  are  two  entirely 
different  kinds  of  seals,  the  eared  seals,  or  sea- 
lions  and  sea-elephants,  in  which  the  hind-limbs 
are  bent  and  are  used  for  progression  on  land, 
and  the  true  seals,  in  which  the  hind-limbs  are 
extended  stiffly  backwards  and  are  useless 
except  for  swimming.  In  these  animals  swim- 
ming is  carried  out  by  lateral  undulations  of 
the  trunk-musculature.  In  this  case,  as  in 
the  whale,  the  ichthyosaurs,  the  crocodiles  and 
the  snakes,  we  find  that  the  means  of  locomo- 
tion are  essentially  the  same  as  those  which 
are  employed  by  fish. 

INFLUENCE  OF  TEMPERATURE  ON  MOVE- 
MENT.— Adaptive  locomotion  influences  every 
part  of  the  body  and  is  in  turn  determined  by 
almost  every  part:  and  in  no  way  is  this  more 
felt  than  in  the  adaptation  to  resist  changes  of 
temperature.  The  lower  vertebrates,  like  the 
invertebrates,  show  very  little  adaptation  to 


MOVEMENTS  OF  ANIMALS          61 

attain  this  desired  end.  It  has  been  found 
that  fish,  for  example,  breathe  less  frequently 
in  cold  wintry  weather  than  in  summer,  and 
are  consequently  very  inactive.  They,  how- 
ever, make  amends  for  inability  to  effectively 
resist  changes  of  temperature  by  migrating 
into  deeper  water,  when  the  local  conditions 
become  unfavorable.  On  land  and  in  fresh 
water  such  changes  are  naturally  more  acute, 
and  we  find  few  animals  that  can  prolong  their 
activity  through  winter  and  summer  alike. 
Frogs  and  reptiles  fall,  as  the  temperature  falls, 
into  a  winter  sleep,  insects  disappear  and  the 
hum  of  summer  life  becomes  inaudible.  Such 
long  periods  of  inanition  are  great  drawbacks 
to  animal  prosperity,  and  it  is  a  significant  fact 
that  the  two  most  highly  organized  and  suc- 
cessful classes,  birds  and  mammals,  are  the 
two  which  are  warm-blooded  and  able  to  main- 
tain their  activity  when  their  more  changeable 
relatives  are  in  profound  stupor. 

The  factors  that  have  led  to  this  desirable 
result  are  very  complex.  Not  only  must  the 
heat  of  the  body  be  retained  by  some  non-con- 
ducting material  such  as  blubber,  hair,  feathers 
or  clothes,  but  there  must  be  some  regulating 
mechanism  to  maintain  that  constant  internal 
temperature  which  is  such  a  marked  peculiarity 
of  these  warm-blooded  creatures.  No  doubt 


62  THE  ANIMAL  WORLD 

there  are  some  fish,  such  as  the  tunny,  just  as 
there  are  some  insects  (bees)  and  snakes  (Py- 
thon), which  can  raise  their  temperature  above 
that  of  the  water  or  air  in  which  they  live,  but 
they  cannot  regulate  the  heat  so  as  to  remain 
uniformly  warm  in  an  inconstant  medium. 
In  a  low  temperature  all  these  imperfectly 
warm-blooded  animals  became  inactive  and 
may  die.  The  heat-mechanism  of  the  mammals 
and  birds  is  an  internal  and  largely  nervous 
one.  It  consists  in  having  good  heat-producing 
muscles  and  glands,  and  in  possessing  also  the 
heat-regulating  nervous  mechanism  for  govern- 
ing the  production  and  the  conduction  of  the 
heat  produced  by  oxidation.  No  wonder,  there- 
fore, that  in  time  of  severe  weather  even  some 
mammals  are  tested  by  the  change  of  temper- 
ature beyond  the  limit  of  their  nervous  control 
and  fall  into  a  state  of  intermittent  or  pro- 
longed stupor,  when  their  temperature  goes 
down  to  a  few  degrees  only  below  that  of  the 
air.  The  cold-temperate  regions  are  perhaps 
the  most  trying  in  this  regard.  Even  the  arctic 
has  a  less  rigorous  and  rapid  change  and  range 
of  temperature  than  our  own,  whilst  the  warm- 
temperate  and  tropical  zones  are,  on  the  other 
hand,  the  most  uniform.  In  this  comparative 
uniformity  and  variability  of  heat  we  have 
one  factor  in  the  distribution  of  animals  and 


MOVEMENTS  OF  ANIMALS          63 

one  explanation  of  the  behaviour  of  animals 
that  have  migrated  from  the  warmer  to  the 
colder  parts  of  the  world. 

DISTRIBUTION  OF  MARINE  ANIMALS. — The 
most  uniform  medium  both  as  regards  con- 
stitution and  conditions  is  undoubtedly  the 
ocean,  though  there  is  far  more  variability  in 
this  seemingly  monotonous  life  than  appears 
at  first  sight.  If  we  divide  the  sea  into  three 
portions  by  lines  drawn  through  latitude  40° 
north  and  south  of  the  equator,  we  have  a  belt 
of  warm  ocean  water  in  the  Atlantic  and  Pa- 
cific oceans,  while  north  and  south  of  this  lie 
the  temperate  and  the  arctic  caps  of  water. 
Speaking  generally  the  fauna  of  the  warm  belt 
is  distinctive  and  fairly  uniform  in  all  parts  of 
the  world,  whilst  that  of  the  two  caps  is  again 
uniform,  but  quite  different  from  that  of  the 
equatorial  zone.  Many  of  the  fish,  Crustacea, 
and  the  minute  animals  of  the  cold,  northern 
seas  are  met  with  again  on  the  coast  of  Pat- 
agonia and  of  South  Africa.  The  arctic  right 
whale  has  its  counterpart  in  the  southern  right 
whale  of  the  antarctic.  The  movements  of 
all  these  boreal  and  hyperboreal  animals  follow 
the  tracks  of  cold  water:  for  example,  the  arctic 
whales  follow  the  cold  Labrador  current  that 
sets  down  the  Newfoundland  coast.  In  the 
same  way  the  warm-water  fauna  follows  the 


64  THE  ANIMAL  WORLD 

drift  of  the  Gulf  Stream  and  of  other  branches 
of  warm  ocean  currents,  so  that  from  time  to 
time  West  Indian  fruits,  turtles  and  fish  are 
occasionally  stranded  on  our  western  coasts  : 
and  still  more  commonly  on  those  of  south- 
western Europe.  Each  fauna  appears  to  die 
out,  however,  if  transferred  to  sea-water  of  a 


Fig.  HA.  —  Basiliscus:  a  lizard  from  Central  America.  The 
specimen  figured  is  a  male,  and  is  distinguished  by  the 
possession  of  a  high  dorsal  crest  on  the  head,  body,  and 
tail.  (After  Gadow,  X  i-) 

different  density  or  temperature.  The  vast 
majority  of  marine  animals  have  no  power 
of  altering  their  constitution  to  suit  water  of 
a  new  degree  of  salinity.  Their  movements 
are  controlled  so  as  to  keep  them  in  the  stream 
or  drift  of  that  kind  of  water  to  which  they  are, 
as  it  were,  attuned,  and  a  very  slight  change 
in  the  water  is  fatal  either  to  themselves  or  to 
their  offspring. 


MOVEMENTS  OF  ANIMALS         65 

Marine  animals  are,  on  the  whole,  of  such 
delicacy  of  constitution,  of  such  nicety  of  dis- 
cernment as  to  need  and  to  keep  in  the  stream 
or  body  of  water  which  only  a  qualified  chemist 
is  able  to  detect  as  differing  from  the  rest  of 
the  ocean  in  which  it  lies  as  a  layer  or  a  deep 
isolated  tongue.  It  is  therefore  a  matter  of 
wonder  that  fresh-water  and  terrestrial  animals 
should  be  able  to  resist  gradually  the  still  more 
pronounced  fluctuations  of  temperature  that 
occur  in  their  environments,  not  only  by  day 
and  night,  but  those  seasonal  changes  of  dry 
and  wet  monsoons,  of  winter  and  summer, 
and,  above  all,  those  still  greater  secular  changes 
in  climate  of  which  the  Ice  Age  is  the  best  known 
example.  If  animals  generally  can  best  live, 
move  and  work  out  their  life-histories  in  a 
fairly  uniform  climate,  then  the  tropics  and 
warm-temperate  countries  would  seem  to  be 
the  most  suitable  ones  for  them,  and,  in  fact, 
the  distribution  of  terrestrial  life  shows  that 
the  richest  fauna  is  that  of  such  countries.  Not 
only  do  the  tropical  and  sub-tropical  lands 
possess  the  most  numerous  and  the  most  varied 
animal  life,  but  they  appear  to  have  furnished 
the  cold-temperate  countries  with  most  of  their 
animals.  Our  reptiles  are  an  impoverished 
group  of  the  abundant  European  snakes  and 
lizards,  and  these  in  turn  are  but  a  section  of 


66  THE  ANIMAL  WORLD 

the  tropical  Oriental  reptilia.  In  the  same 
way  tropical  South  and  Central  America  is  the 
zone  of  the  new  world  most  favourable  for 
poikilothermic  animals,  that  is,  such  as  are  of 
a  variable  temperature,  and  as  we  proceed 
northward  into  the  states  or  southward  into 
the  republics  the  variety  of  animal  life  dimin- 
ishes until  the  arctic  in  the  one  case  and  the 
inhospitable  Patagonian  lands  in  the  other, 
contain  few  animals  except  the  mammals  and 
birds,  that  are  able  to  withstand  by  their  ho- 
moiothermic  mechanism  (or  constant  temper- 
ature) the  rigours  of  cold  and  of  change. 

This  long  explanation  has  been  needed  in 
order  to  suggest  why  it  is  that  the  ability  to 
support  life  and  to  flourish  in  a  country  that 
changes  much  during  the  year,  has  enabled 
birds  and  mammals  to  supplant  reptiles  and 
to  assume  the  premier  position  amongst  animals. 
Birds  have  excelled  reptiles  in  virtue  of  their 
constant  internal  warmth.  This  enables  them 
to  range  over  the  whole  world,  to  remain  active 
throughout  the  year,  to  rear  their  young  quickly, 
and  therefore  to  become  subject  to  a  far  greater 
range  of  experience  than  can  fall  to  the  lot  of 
their  more  sluggish  and  more  tropic-bound 
relations — the  tortoises,  crocodiles  and  lizards. 
In  the  same  way,  mammals  have  excelled  rep- 
tiles by  reason  of  the  same  homoiothermic 


MOVEMENTS  OF  ANIMALS         67 

faculty,  but  in  their  case  the  facilities  for  lo- 
comotion are  more  limited.  The  superiority 
of  mammals  is  based  on  their  accumulation 
of  modifiable  experience  or  educability,  on 
their  association  in  flocks  or  herds,  and  on  their 
size  and  strength.  In  either  case  the  dominance 
of  birds  and  mammals  was  easy  in  the  cold- 
temperate  and  arctic  lands,  for  these  could 
not  have  been  peopled  by  reptiles,  and  could 
only  support  animals  that  were  able  to  with- 
stand severe  fluctuations  of  climate. 

Nevertheless  these  reasons  are  not  sufficient 
to  explain  why  the  great  reptiles  died  out,  for 
it  seems  probable  that  some  of  them  (the  flying- 
reptiles  and  the  whale-like  forms)  had  actually 
evolved  a  mechanism  for  maintaining  a  con- 
stant bodily  heat  and  were  therefore  as  inde- 
pendent of  their  environment  as  birds  or  whales 
are  to-day.  The  same  difficulty  meets  us  in 
explaining  the  disappearance  of  any  highly- 
organized  system.  Lack  of  brains,  lack  of 
adaptability,  some  innate  tendency  to  regress 
after  high  specialization  had  been  attained, 
competition  with  other  rivals,  impaired  birth- 
rate, these  and  other  hypotheses  have  all  a 
certain  weight  but  fail  as  a  general  explanation 
of  extinction.  We  see  the  passing  of  a  race 
apparently  built  for  eternity  and  "all  that 
mighty  heart  is  lying  still.551 


68  THE  ANIMAL  WORLD 

SUMMARY. — A  summary  of  the  chief  factors 
in  the  history  of  animal  locomotion  would  in- 
clude the  following.  For  sustained  movement 
a  certain  constancy  either  in  the  medium 
around  animals  or  in  the  temperature  of  the 
animal  itself,  is  requisite.  Hence  the  sea  is 
the  most  favourable  medium  for  most  animals. 
On  the  land,  predominance  is  assured  for 
those  groups  (birds  and  mammals)  which  have 
succeeded  in  forming  an  inner,  hot  medium 
independent  of  the  temperature  of  their  sur- 
roundings. This  constant  temperature  is,  how- 
ever, only  sustained  by  plentiful  food — hence 
innumerable  devices  to  ensure  its  up-keep. 
Migration  is  one  of  these  devices,  e.  g.  the  north- 
ward spring  migration  of  many  birds  in  quest 
of  food  and  nesting  sites.  In  autumn  these 
birds  migrate  southwards,  returning  to  the 
warm  countries  in  which  life  is  then  easier  to 
sustain  than  in  the  changing  climate  of  the 
temperate  zones.  Warmth  then  assists  birds 
as  it  does  to  a  greater  extent  the  cold-blooded 
animals.  Movement,  therefore,  partly  explains 
the  distribution  of  animals. 

For  the  great  majority  of  animals  driven 
by  competition  to  adopt  fresh  water  or  terres- 
trial life,  the  seasonal  changes  of  temperature, 
of  food,  and  of  moisture  lead  to  intermittent 
movement.  Winter,  drought,  lack  of  food  cause 


MOVEMENTS  OF  ANIMALS  69 

sluggishness  and  stupor  if  not  decimation  of 
the  summer  brood.  Hence  the  lack  of  high 
organization  in  many  groups:  and  hence  also 
a  vast  array  of  adaptive  measures  for  protection 
to  old  and  young  during  the  testing  season 
(for  example,  burrows  in  earth,  manure,  etc., 
adherence  to  man's  dwellings,  cave-life,  moss- 
life).  These  contrasts  are  least  felt  in  the 
tropical,  sub-tropical  and  warm-temperate 
countries,  and  it  is  here  that  animal  life  varies 
more  and  attains  higher  development  than 
further  north  and  south.  The  temperate  zones 
are  colonized  chiefly  by  enterprise  from  the 
warmer  countries. 

Movement  is  a  property  of  being.  Its  effec- 
tiveness reacts  on  the  whole  body.  Movement 
in  a  definite  direction,  implying  a  "head," 
right  and  left  sides,  and  a  creeping  surface,  is 
first  seen  in  the  Planarians  and  their  allies: 
effective  movement  in  the  segmented  worms 
and  Mollusca  :  sustained  movement  in  insects 
and  fish.  The  evolution  of  terrestrial  animals 
must  have  occurred  several  times  and  led  to  the 
Amphibia,  Reptilia,  birds  and  Mammalia.  The 
Amphibia  and  Reptilia  are  probably  sister- 
stocks,  the  first  still  leading  the  life  of  the  swamp, 
the  second  emerging  into  the  open  and  even 
taking  to  flight.  The  reptiles  exhibit  all  the 
modes  of  locomotion  and  have  even  gone  back 


70  THE  ANIMAL  WORLD 

to  the  rivers  and  seas.  Their  age  of  mastery 
is  a  thing  of  the  past,  though  it  is  inexplicable 
why  they  succumbed.  Birds  are  the  most  facile 
of  all  animals  in  adopting  a  suitable  change 
of  environment  by  migration  or  in  surviving 
a  changing  one  by  their  almost  omnivorous 
habit,  and  are  shielded  in  either  case  by  their 
constant  temperature.  In  mammals  the  brain 
has  developed  far  beyond  that  of  other  animals 
or  even  their  muscular  and  governing  needs. 
They  and  they  alone  appear  to  be  able  to  ac- 
cumulate and  modify  their  experience  and  to 
succeed  in  establishing  themselves,  not  so  much 
by  following,  like  birds,  the  line  of  least  resist- 
ance, as  by  subtle  adaptations  which  enable  them 
to  hold  their  outposts  and  to  endure  both  the 
arctic  and  tropic  without  flinching. 


CHAPTER  III 

THE    QUEST    FOR    FOOD 

"ALL  things  flow,"  said  old  philosophy,  and 
indeed  the  appearance  of  stability  in  nature 
scarcely  hides  essential  impermanence  and  fluc- 
tuation. The  mountains  fall,  worn  down  by  ice, 
water,  and  the  attrition  of  ages;  new  ranges 
arise,  are  piled  up  by  the  sea  on  rising  coasts  and 


THE    QUEST    FOR    FOOD  71 

lifted  beyond  the  action  of  waves  into  that  of 
frost  and  snow  by  the  crumpling  of  the  earth's 
crust.  Material  and  energy  are  building  up  and 
pulling  down  the  furniture  of  the  earth,  yet  so 
slowly  that  their  action  seems  ineffectual. 

METABOLISM. —  Organic  nature  is  more  pal- 
pably impermanent.  The  may-fly  endures  for 
a  day,  a  thread-worm  for  a  week,  a  planarian  for 
a  month,  a  worker  bee  for  four  months,  a  shrew 
and  a  pellucid  goby  for  a  year,  a  coral  for  twenty- 
five  years,  a  man  for  seventy  years,  a  bird  for  a 
hundred  years.  Each  moment  of  these  lives  is 
filled  with  a  double  action:  one  phase  destruc- 
tive, the  other  constructive,  and  life  is  the  out- 
come of  the  balance  between  these  two  phases  of 
action.  As  in  the  case  of  mountains  there  are 
the  matter  and  the  energy  to  be  considered:  and 
the  problem  of  life  is,  first,  what  matter  is  lifted 
to  the  point  of  living,  and  second,  what  forces 
lift  it  to  that  point  and  then  hurl  it  down  in  order 
to  give  place  to  its  successors.  The  seeming 
stability  of  an  inert  man  or  reptile  is  deceptive, 
like  that  of  a  volcano.  There  is  hidden  beneath 
the  placid  exterior  a  successional  flow  of  construc- 
tive and  destructive  events,  and  the  placidity 
is  due  to  the  enormous  reserves  of  force  and  of 
material.  The  mountain  of  flesh  is  wearing  down, 
but  like  the  mountain  of  rock  the  process  is  slow. 
As  each  range  of  tissue  goes  silently  to  its  death 


72  THE   ANIMAL   WORLD 

it  is  renewed  by  fresh  stores  of  material  from  the 
reserve  funds  of  fat  or  starch  or  egg-white.  These 
larders  are  of  generous  size  and  are  distributed  in 
many  ways;  but,  long  before  they  are  depleted, 
the  body  becomes  hungry  and  cries  for  more,  as 
though  aware  of  working  near  the  breaking  strain. 
Then  out  of  a  winter  sleep  of  months  or  the  noc- 
turnal sleep  of  hours,  the  body  wakes  and  casts 
about  to  renew  its  larders. 

The  quest  for  food  is  thus  involved  in  the  very 
constitution  of  matter  and  the  wearing  attribute 
of  energy,  and  is  enhanced  by  the  fact  that  we 
bank  our  income  and  draw  upon  our  deposit  by 
small  drafts.  This  system  provides  against  the 
great  day  of  adversity;  enables  the  winter  sleeper 
to  survive  its  long  stupor;  the  migrant  to  make  its 
long  marches;  the  hot-blooded  to  keep  its  tem- 
perature even  when  food  fails  for  days.  The 
most  important  of  all  such  drains  on  the  reserve 
fund  of  nourishment  is  the  feeding  of  the  young. 
The  egg,  full  of  meat,  is  the  symbol  of  organic  life 
in  its  provident  reserve,  latent  capacity  and  in- 
dependence. 

But  not  only  is  food-material  dissolved,  built 
up  into  living  substance  and  analyzed  into  waste 
material,  there  must  concomitantly  be  a  cycle  of 
energetic  processes.  Food,  animal  food,  is  energy 
stored  in  material  that  has  embodied  an  earlier 
system  of  such  changes.  It  not  only  replenishes 


THE  QUEST  FOR  FOOD  73 

the  worn-out  material,  but  it  reinforces  waning 
energy.  This  fresh  supply  is  partly  reserved, 
partly  used  for  current  emergencies.  It  is  banked 
with  the  reserve  food-supply  and  it  may  evolve 
heat.  If  the  animal  is  a  homoiothermic  one,  it 
will  require  more  continuous  supplies  of  food  than 
a  cold-blooded  one  in  order  to  maintain  the  con- 
stant temperature,  to  replace  the  vast  quantities 
of  material  that  go  to  waste,  and  also  the  expen- 
diture laid  out  in  eggs  and  milk. 

IMPORTANCE  OF  PLANTS. — From  the  point  of 
view  of  biological  economics  there  are  three 
sources  of  food.  Plants  (including  bacteria)  fur- 
nish the  first  of  these;  organic  fluid  media  (either 
the  result  of  plant-action  in  water  or  the  fluids 
of  animals)  the  second;  and  other  living  animals 
the  third;  that  is,  vegetables,  drinks  and  flesh. 
In  all  cases  the  plant-world  furnishes  the  basis, 
and  we  may  say  that  animals  depend  absolutely 
on  plants  for  their  renewal  of  matter  and  energy. 
To  understand  animal  economics,  therefore,  we 
must  glance  for  a  moment  at  the  abundance  and 
distribution  of  plants. 

The  basal  and  omnipresent  forms  of  plant  life 
are  bacteria,  blue-green  algse,  and  their  allies, 
the  yeasts  and  moulds.  Bacteria  occur  in  all 
seas  and  in  all  soils.  In  a  moist,  warm  soil  there 
may  be  50,000,000  per  gramme,  and  the  whole 
fertility  of  nature  above  ground  and  under  water 


74  THE  ANIMAL  WORLD 

depends  on  these  microscopic  organisms.  Blue- 
green  algae  are  the  first  organisms  to  populate 
devastated  regions,  the  first  to  form  soil  on  rock 
and  to  render  the  barren  land  fertile.  Yeasts  are 
minute  bodies  that  lie  scattered  over  the  earth 
and  trees  and  that  are  blown  into  our  houses  as 
dust.  Yeasts  and  bacteria  break  down  organic 
solutions  or  bodies,  and  are  able  to  set  up  exten- 
sive changes  leading  from  organic  to  inorganic 
substances  (chiefly  carbonic  acid  and  water)  with- 
out undergoing  any  appreciable  loss  of  material 
or  energy.  These  are  the  sensitizers  of  nature, 
the  pullers  of  triggers,  as  it  were,  that  by  their 
mere  presence  set  going  vast  exchanges  of  energy 
and  alterations  of  chemical  structure.  Then  we 
have  other  moulds  that  live  in  water  and  on  land, 
forming  networks  of  delicate  strands  in  every 
fertile  soil  and  densely  massed  around  the  roots 
of  trees,  lilies  and  many  other  plants.  Their 
spores  are  in  every  atmosphere,  and  we  have  only 
to  let  a  room  remain  unused  or  woodwork  to 
become  damp  in  order  to  discover  the  mouldy 
smell  or  the  dry  rot  that  have  been  kept  at  bay 
during  the  more  hygienic  regime.  Then  we 
come  to  the  green  plants  that  can  only  live  in 
light,  but  that,  given  light,  can  synthesize  water 
(the  complex  water  of  nature,  not  extracted 
"pure"  water)  and  carbon  dioxide  into  an  alde- 
hyde or  simple  form  of  carbon  compound.  This 


THE   QUEST    FOR    FOOD  75 

compound  is  then  converted  into  a  form  of  sugar, 
starch  or  oil,  and  ultimately  this  is  built  up  into 
a  living  molecule.  In  the  sea  and  in  fresh  water, 
millions  of  such  green  plants  live  alone  or  in 
chains,  such  as  those  minute  algae  which  discolour 
our  garden  paths  or  farmyards.  Others,  form- 
ing a  combination  with  certain  moulds,  produce 
the  dual  organism  so  familiar  under  the  form  of 
lichens  that  cover  the  uplands  of  Scandinavia 
and  that  eat  into  the  very  stones  of  the  limestone 
country.  Still  more  important  from  the  point 
of  view  of  animal  provender  is  the  multitude  of 
diatoms,  unicellular  alga?  enclosed  in  a  sculp- 
tured flinty  case  or  box,  which  swarm  in  both 
sea  and  lake,  often  discolouring  the  water. 

CAPTURE  OF  SIMPLE  PLANTS. — Upon  this 
prevalent,  minute  plant  life  in  sea,  pond  and  soil 
animals  depend  for  their  food.  All  that  is  neces- 
sary for  its  capture  is  the  provision  of  a  throat 
capable  of  gulping  down  incessant  draughts  of 
water  or  mouthfuls  of  soil,  of  straining  off  the 
plants  and  of  discharging  the  rest.  Active 
search  is  hardly  required,  and  in  fact  the  great 
majority  of  such  feeders  are  fixed  or  move  but 
little.  The  Protozoa,  many  hydroids,  jelly-fish 
and  corals,  the  lower  Crustacea,  especially  the 
Copepods,  Barnacles  and  Phyllopods  (the  group 
of  which  Daphnia  is  a  member),  the  sponges  and 
various  encrusting  forms  of  animal  life  feed  in 


76  THE   ANIMAL   WORLD 

this  way.  Bivalves  and  lamp-shells  inhale  a 
ceaseless  current  of  water  for  the  same  purpose, 
whilst  the  majority  of  snails  and  slugs  browse 
upon  algse  or  higher  forms  of  plant  life.  Soil, 
whether  sea  soil  or  land  soil,  being  tenanted  by 
bacteria  and  fungi,  is  swallowed  in  great  masses 
by  earth-worms,  sea-worms,  and  sea-cucumbers. 
This,  then,  is  the  first  and  simple  method  of  ani- 
mal nutrition:  drinking  the  water  and  eating 
the  soil.  On  such  a  simple  diet,  obtained  with 
so  little  exertion,  the  lower  orders  of  all  the  great 
phyla  or  divisions  depend. 

HERBIVOROUS  HABITS. — The  gradual  devel- 
opment of  land  plants,  however,  sets  up  certain 
difficulties  in  the  way  of  a  purely  vegetarian 
diet  for  terrestrial  animals.  Aquatic  plants,  and 
especially  algae,  are  comparatively  easy  of  diges- 
tion, but  land  plants,  in  order  to  withstand  their 
own  weight  and  the  forces  of  wind,  have  developed 
stouter  investments,  e.  g.  coarser  vessels,  woody 
fibres,  and  these  adaptations  cause  much  plant 
food  to  be  relatively  indigestible.  Hence  we 
find  that  herbivorous  habits  are  less  commonly 
the  rule  amongst  the  lower  terrestrial  animals 
than  amongst  their  aquatic  allies,  and  that  a 
choice  is  often  made  of  the  more  primitive  and 
less  hardened  plants  and  of  the  more  succulent 
ones  amongst  newer  kinds.  Ferments  for  dis- 
solving the  cuticle  of  leaves  have  been  evolved, 


THE  QUEST  FOR  FOOD  77 

and  in  special  cases  these  are  so  effective  that 
wood-boring  insects  of  many  orders  are  known. 
But  the  difficulty  of  vegetarian  diet  is  well  shown 
in  the  mammalia,  for  it  is  chiefly  the  Rodents 
and  the  Ungulates,  with  their  numerous  and 
complicated  grinders  and  their  frequently  com- 
plex stomach,  that  are  able  to  subsist  entirely 
on  the  shoots  of  plants.  All  other  vegetarians 
are  either  only  partially  so  or  select  chiefly  the 
fruit  and  seeds,  to  the  exclusion  of  the  tougher 
leaves  and  wood. 

Of  these  herbivorous  land  animals,  insects, 
especially  insect-larvse,  and  molluscs,  birds  and 
mammals  form  the  chief  portion.  There  are,  it 
is  true,  aquatic  insects  which  pass  the  whole  or 
all  but  their  final  stage  of  existence  in  water, 
but  these  form  an  insignificant  section  of  the 
vast  class.  Probably  half  a  million  different 
kinds  of  insects  are  terrestrial;  the  most  numer- 
ous being  the  Lepidoptera,  the  beetles  and  two- 
winged  flies;  ants,  though  not  so  varied,  are 
immensely  numerous  individually.  Each  of  these 
half  million  insects  required  one  kind  of  food 
when  young  and  usually  a  perfectly  different 
kind  when  grown  up.  The  food  plants  of  the 
larvae  are  usually  well  defined.  The  common 
white  butterflies  select  Cruciferae;  the  "blues" 
select  violas;  the  white  "ants,"  or  termites, 
wood.  The  most  varied  methods  have  been 


78  THE  ANIMAL  WORLD 

adopted  for  comminuting  the  food  in  order  that 
the  dissolving  ferments  may  work  effectively 
upon  it.  Thus  jaws  become  a  necessity,  and  in 
most  insects  there  are  three  pairs  of  such  which 
work  from  side  to  side  and  not  like  our  own, 
from  below  upwards.  The  first  pair,  or  mandi- 
bles, are  the  most  effective,  the  second  and  third 
serving  as  a  sort  of  underlip.  By  the  aid  of 
these  mandibles  and  maxillae,  as  they  are  termed, 
fungi,  lichens,  leaves,  and  even  the  wood  itself 
are  gradually  devoured.  A  few  groups  of  wild 
plants  appear  to  be  exempt  from  attack,  Ferns 
and  their  allies  generally  being  the  most  noted 
exceptions,  but  when  grown  in  captivity  or 
transferred  from  their  native  soil  to  gardens 
they  often  lose  their  power  of  resistance  or  quality 
of  repugnance. 

INTERACTION  OF  PLANTS  AND  INSECTS. — 
Plants,  however,  offer  another  and  still  more 
attractive  source  of  nutriment:  the  sap  and  the 
pollen.  For  some  reason,  not  thoroughly  under- 
stood, these  sweet  and  delicate  foods  are  rarely 
sought  after  by  larvae.  Sap  and  honey  form  the 
food  and  drink  of  most  winged  insects  in  their 
final  phase.  Pollen  is  the  food  of  bee-larvae  and 
of  some  wasps.  To  gather  this,  far  more  com- 
plicated mechanism  is  essential  than  a  mere 
chewing  arrangement.  A  tube  and  a  suction- 
pump  have  to  be  evolved  in  order  to  drink  the 


THE  QUEST  FOR  FOOD  79 

sap  and  baskets  to  carry  the  pollen.  Hence  we 
find  that  the  mouth  and  jaws  of  most  mature 
insects  are  differently  and  more  highly  organized 
than  those  of  mandibulate  orders  and  of  larvae. 
In  butterflies  and  moths  the  first  maxillae  are 
drawn  out  into  a  highly  sensitive  trunk.  In 
bees,  flies  and  other  orders  the  second  maxillae 
are  similarly  modified.  Such  exquisite  adapta- 
tion has  taken  long  ages  for  its  execution.  Hand 
in  hand  with  it  has  gone  a  remarkable  finesse  in 
structure  generally.  The  brain  in  such  insects 
is  enlarged  and  lobed;  the  eyes  grow  by  addition 
of  new  facets  throughout  life;  the  power  of  flight 
is  perfect;  the  muscular  control  sustained  and 
delicate. 

INSECT-FERTILIZERS. — In  their  search  for  the 
nectaries  of  honeyed  flowers  such  insects  per- 
form memorable  service  to  the  plants  they  visit. 
As  a  bee  enters  into  a  foxglove  or  snapdragon  it 
brushes  against  the  anthers  that  lie  along  the 
hooded  petal,  and  in  doing  so  dusts  itself  with 
pollen.  As  it  works  down  the  column  of  flowers 
it  brings  the  pollen  from  one  against  the  stigma 
of  another  and  cross-fertilizes  it.  The  result  is 
that  such  plants  as  a  rule  bear  more  and  better 
seed  than  self -fertilized  flowers  of  the  same  plant. 
In  fact,  many  plants  take  pains,  as  it  were,  to 
avoid  self-fertilization  and  to  ensure  the  visits 
of  insects.  Flower  and  insect  have  reacted  upon 


80  THE   ANIMAL   WORLD 

one  another;  the  flower  acquiring  odour,  streak- 
iness  of  colouring,  modifications  of  shape  and 
size,  rhythmical  opening  by  day  or  by  night;  the 
insect  developing  a  complex  "tongue,"  hairy 
body  and  legs,  acute  sense  of  smell  and  probably 
of  sight.  The  correspondence  between  the  two 
sets  of  modifications  may  be  so  close  that  one 
insect  only  serves  to  fertilize  a  particular  flower, 
as  the  Pronuba-moth  that  effects  the  crossing 
of  the  Yucca-palm;  and  a  certain  wasp  in  the 
case  of  the  Indian  fig.  Further  information  on 
this  fascinating  subject  should  be  read  in  Dar- 
win's classic  and  the  recent  work  of  Knuth. 

INSECT-PARASITES. — Another  method  of  ob- 
taining the  sap  from  plants  is  adopted  by  the 
immense  group  of  Hemiptera,  the  order  of  in- 
sects to  which  Aphides,  Frog-hoppers  or  Cuckoo- 
spit  insects  and  Scale  insects  belong.  A  similar 
method  is  practised  by  many  dipterous  insects. 
It  consists  in  the  puncture  of  plants  by  the  aid 
of  a  stylet  or  group  of  stylets,  and  the  subsequent 
sucking  of  the  sap  into  the  narrow  tubular  throat. 
The  Hemiptera  possess  a  pair  of  second  maxillae 
specially  modified  to  form  a  lancet-like  instru- 
ment or  a  piercing  hollow  spine.  The  mosqui- 
toes, on  the  other  hand,  have  three  pairs  of  slender 
stylets  which  cut  a  wound  and  inhale  the  sap. 
Such  insects  abound  in  all  countries,  and,  being 
able  to  find  sufficient  nourishment  in  a  single 


THE  QUEST  FOR  FOOD  81 

plant  both  for  themselves  and  for  several  broods, 
they  often  acquire  an  extremely  sluggish  dispo- 
sition, either  dispensing  with  wings  altogether  or 
developing  them  only  in  the  case  of  the  male 
or  of  certain  broods.  Thus  the  scale  insects 
which  are  common  on  oranges,  lemons,  tomatoes 
and  other  plants  are  immobile;  the  green  fly 
rarely  move,  and  drift  rather  than  fly  over  the 
countryside.  Mosquitoes,  however,  and  other 
piercing  Diptera,  have  retained  their  active 
habits,  and  large  companies  of  males  may  often 
be  seen  performing  complex  evolutions  over  a 
river  or  forming  a  dense  cloud  over  the  country. 
The  food  of  these  swarms  is  still  little  known,  but 
it  seems  probable  that  it  differs  from  that  of  the 
more  sluggish  female  mosquito  since  the  male  has 
fewer  piercing  organs.  Either,  therefore,  he, 
like  many  other  male  animals,  takes  no  food  and 
lives  only  a  short  time,  relying  for  sustenance 
upon  the  reserves  accumulated  during  larval 
life,  or  he  sucks  with  his  feebler  mouth-parts  the 
sap  that  exudes  from  injured  trees  or  from  the 
nectaries  of  flowers. 

BLOOD-SUCKING  INSECTS. — This  difference  be- 
tween the  piercing  mouth  of  the  female  mosquito 
and  the  simpler  mouth  of  the  male  is  the  prelude 
to  a  tragic  episode  in  animal  life  and  in  human 
life.  In  all  insects  the  production  of  eggs  in 
rapidly  recurring  clutches  causes  a  great  drain 


82  THE  ANIMAL  WORLD 

upon  the  reserve-material  of  the  female,  and  it  is 
advantageous  to  her  and  to  her  race  if  a  more 
stimulating  or  more  abundant  food  can  be  sub- 
stituted for  a  less  nutritious  one.  This  advan- 
tage is  emphasized  if  such  food  can  be  obtained 
during  the  spring  and  summer,  since  heat  deter- 
mines the  period  of  egg-production  and  hastens 
the  development  of  the  young.  Mosquitoes  and 
other  insects  have  discovered  that  stylets  will 
pierce  hide  as  well  as  bark,  and  that  warm  blood 
is  a  food  combining  the  advantage  of  stimulating 
nourishment  with  that  of  warmth.  Hence  the 
origin  of  the  blood-sucking  habit.  Such  a  habit, 
however,  carries  with  it  many  complications. 
If,  for  example,  the  blood  of  the  host  is  charged 
with  protozoa,  the  female  mosquito  becomes  in- 
fected by  protozoa,  and  when  sucking  the  next 
host  transfers  some  of  the  parasites  to  that  host 
and  so  infects  it.  Blood-sucking  insects,  mites 
and  ticks,  have  thus  come  to  be  regarded  as  one 
of  the  most  dangerous  sources  of  dissemination 
of  disease  and  the  only  means  of  spreading  such 
appalling  inflictions  as  sleeping-sickness,  yellow 
fever,  certain  kinds  of  plague,  typhus  fever,  black- 
water  fever  and  other  tropical  diseases.  The 
subject  is  too  extensive  to  be  dealt  with  here,  but 
full  references  can  be  obtained  in  Sir  Rupert 
Boyce's  recent  works. 
THE  CARNIVOROUS  HABIT.  — The  quest  for 


THE  QUEST  FOR  FOOD  83 

food  has  led  many  groups  of  animals  to  adopt 
a  carnivorous  diet. 

The  habit  probably  began  in  the  sea,  since 
the  floating  and  drifting  life  (plankton)  of  the 
ocean  is  not  only  composed  of  minute  algae  and 
bacteria,  but  of  the  myriads  of  small  or  young 
animals  that,  as  we  have  seen,  feed  upon  this 
fundamental  vegetable  soup.  Hence  the  filtrate 
left  stranded  on  the  throat  of  a  plankton-feeder 
is  partly  animal  and  partly  vegetable.  More- 
over, certain  plankton  feeders,  particularly  Cope- 
pods,  occur  in  such  swarms  as  to  attract  the 
attention  of  many  fish.  However  the  habit  has 
been  evolved  it  is  certainly  a  very  old  one  and 
has  probably  been  re-discovered  and  adopted 
time  after  time  in  animal  history. 

CIRCULATION  OF  FOOD. — Briefly  we  may  re- 
capitulate the  circulation  of  food  in  the  sea  or 
lake  as  follows.  First,  bacteria  capable  of  fixing 
nitrogen  and  living  upon  inorganic  compounds 
but  creating  organic  ones.  Then,  bacteria  living 
upon  these  organic  compounds  and  exuding 
others;  algse  needing  light,  carbonic  acid,  and 
salts,  but  flourishing  better  in  an  organic  medium, 
or  at  least  one  with  a  certain  quantity  of  nitrates, 
produced  in  turn  by  the  bacteria.  In  such  a 
medium  diatoms  abound  and  microscopic  algae 
multiply  exceedingly,  and  upon  them  animals 
of  the  most  varied  kinds  depend  for  food:  the 


84  THE  ANIMAL  WORLD 

young  or  larval  stages  of  nearly  all  marine  ani- 
mals requiring  this  diatom  and  algal  food  and 
rejecting  all  others.  It  is,  to  them,  their  milk, 
without  which  they  cannot  grow  or  develop. 
Thus  the  shores  and  the  high  seas  are  full  of 
thirsty  throats,  drinking  the  sea,  exhaling  the 
filtered  water  and  swallowing  the  residue.  These 
delicate  feeders  are  themselves  in  turn  the  food 
of  others.  Fish,  such  as  sole,  feed  upon  bivalves; 
the  mackerel,  mullet  and  herring  on  Copepods; 
whalebone  whales  engulf  swarms  of  Pteropods 
or  sea-butterfiies  (floating  molluscs  of  the  high 
seas)  and  pelagic  Tunicates  or  sea-squirts. 
Then  fish  themselves  are  preyed  upon  by  larger 
forms.  Man  himself  is  their  greatest  enemy; 
dogfish  and  sharks,  angler-fish  and  wolf-fish, 
are  the  carnivores  of  the  sea.  Gulls  and  gannets, 
puffins  and  guillemots  consume  vast  numbers 
of  fish-fry,  smelts,  pilchards  and  sprats. 

In  a  similar  manner  the  cycle  of  economics 
is  completed  on  land.  Bacteria  are  the  source 
of  soil-fertility — the  whole  of  agriculture  is 
dependent  upon  the  abundance  of  these  invisible 
particles.  Soil  produces  vegetation — algse,  fun- 
goid, yeasty,  cryptogamic  (ferns,  mosses)  and 
phanerogamic  (pines  and  flowering  plants).  In 
soil  and  close  to  the  ground  are  the  primitive 
vegetarians — the  simple,  flightless  insects  (silver- 
fish,  beetles,  ants),  the  millipedes,  the  earth-  and 


THE  QUEST  FOR  FOOD  85 

tree-worms,  the  snails  and  slugs.  These  cannot 
simply  drink  their  food  like  their  representatives 
in  the  sea:  they  have  to  seek  it,  and  they  need 
jaws  to  cut  it  up.  More  highly  organized  than 
these  are  the  vegetarians  that  devour  leaf  and 
fibre,  that  suck  nectaries  and  that  puncture  the 
wood- vessels  in  order  to  drink  the  sap;  that 
rob  our  orchards  and  hedgerows  of  fruit  and  seed 
and  split  acorn  and  pine-cone  for  the  kernels. 
These  two  classes  of  vegetable  feeders  constitute 
a  vast  assemblage  of  animals,  whilst  a  large 
number  are  carnivorous,  especially  in  spring. 
The  mole,  the  thrush-family  and  redbreast  devour 
the  primitive  worms.  Tits,  migrants  and,  in 
fact,  land  birds  of  all  kinds  are  insectivorous 
during  the  nesting  season.  Ant-eaters,  shrews, 
bats  and  bears  depend  largely  on  an  insect  diet- 
ary. Insects,  in  fact,  correspond  economically 
to  that  vast  host  of  delicate  feeders  of  the  sea; 
and  larval  insects,  being  especially  easy  of  di- 
gestion, are  the  most  attractive  and  fattening 
food  for  nestlings.  Hence  the  great  swarms  of 
migrants  from  the  warm  temperate  countries 
to  cold  temperate  and  arctic  lands  in  the  short 
spring  and  summer;  for  in  north  countries  not 
only  is  the  supply  of  larvae  great  when  in  warmer 
countries  it  is  lessening  but  owing  to  the  light- 
conditions  of  the  North  during  the  summer,  a 
longer  working  day  is  assured  in  which  to  collect 


86  THE  ANIMAL  WORLD 

stores  of  food  to  stay  the  constant  hunger  both 
of  parents  and  of  their  young.  When  the  nest- 
ing season  is  over  many  birds  become  vegetarians 
again  (such  as  sparrows  and  rooks),  but  nearly 
all  the  migrants  are  insectivorous  throughout 
the  year. 

SPIDER-WEBS. — The  habits  of  spiders  require 
especial  mention,  owing  to  their  habit  of  making 
burrows,  sheet-webs  and  wheel-webs,  for  en- 
trapping their  prey.  The  habit  probably  began 
by  the  burrowing  spiders  lining  their  retreat  with 
silk  and  enclosing  their  eggs  with  a  silken  case 
in  order  to  carry  them  about.  Such  spiders  do 
not  spin  webs  but  jump  upon  their  prey  by  night 
and  enclose  it  with  a  gummy  envelope.  These 
spiders  began  hunting  above  ground,  but  still 
constructed  a  tubular  retreat  in  which  they 
lurk  and  from  which  they  rush  upon  their  prey. 
Loose,  irregular  flat-webs  were  probably  the 
next  stage,  and  under  these  the  spiders  lie,  back 
downwards.  Last  of  all  come  the  geometric 
spiders  that  construct  the  orbs  or  wheel-webs 
with  which  we  are  all  familiar. 

RAPACIOUS  ANIMALS. — The  number  and  variety 
of  land  animals  that  feed  upon  others  higher 
than  insects  are  very  limited.  Owls  and  kestrels 
keep  down  the  threatened  plague  of  rats  and 
mice,  even  though  they  may  now  and  again 
carry  off  a  young  bird.  Ravens  and  eagles  are 


HOW   ANIMALS    BREATHE          87 

more  rapacious,  but  are  so  rare  in  this  country 
as  to  do  little  harm  to  lambs  in  most  districts. 
Foxes  and  stoats  are  capable  of  lustful  killing, 
but  these  and  the  larger  carnivora  and  snakes 
are  among  the  comparatively  few  terrestrial 
orders  of  the  animal  kingdom  which  seek  their 
prey,  and  in  contrast  to  them  are  many  vege- 
tarians— the  monkeys,  lemurs,  rodents,  ungu- 
lates, sirenia  (sea-cows);  the  bears  and  badgers; 
all  the  hard-billed  birds;  most  lizards  and  tor- 
toises. 


CHAPTER  IV 

HOW   ANIMALS   BREATHE 

THERE  is  a  true  sense  in  which  food  is  a  source 
of  energy.  Just  as  we  feed  fire  with  coal  or  oil, 
so  the  fire  of  life  has  to  be  fed  with  carbon  in  the 
form  of  starch,  oil  or  egg-white,  and  the  split- 
ting up  of  these  substances  under  the  action  of 
ferments  releases  energy.  Hunger  is  not  only 
a  signal  of  material  distress  but  of  low-tide 
energy:  and  a  meal  which  restores  the  balance 
of  fuel  also  renews  vigour. 

Food  in  itself,  however,  is  rarely  able  to  sus- 
tain the  output  of  energy  due  to  movement, 
heat,  electrical  discharge  and  other  causes  of 
loss.  Fuel  is  effective  in  proportion  to  the 


88  THE  ANIMAL  WORLD 

draught  of  the  grate.  The  oxidation  of  the 
carbon,  oil  or  acetylene  needs  a  current  of  air, 
and  if  we  are  to  obtain  an  output  of  heat,  of 
motor,  and  of  electrical  energy  from  an  animal 
we  must  supply  it  not  only  with  food  but  with 
air.  A  draught  is  needed  for  the  living  body 
as  well  as  for  the  coal-fire.  Breathing  is  the 
means  by  which  the  draught  is  set  up  and  carried 
to  all  the  microscopical  furnaces  of  the  body. 

The  essential  element  in  the  breath  inhaled 
either  directly  from  the  air  or  indirectly  from 
air  dissolved  in  water  (as  in  gill-breathing  crea- 
tures) is  oxygen,  exactly  as  it  is  oxygen  which 
allows  coal  or  wood  to  flame.  The  wood  or 
coal  is  a  mass  of  material  in  which  sun-energy 
is  locked  up  or  banked  up  and  stored.  One 
condition  under  which  it  can  be  made  to  give 
out  that  energy  in  the  form  of  tangible  heat  is 
that  the  carbon  shall  be  made  to  unite  with  the 
oxygen  of  the  atmosphere;  but  we  may  leave 
a  lump  of  coal  in  a  constant  draught  for  years 
and  no  fire  will  occur.  Imagine,  however,  the 
coal  to  be  powdered  into  fine  coal  dust  and  to 
be  placed  in  a  mine  that  is  under  changing  con- 
ditions of  moisture  and  of  heat.  There  is  then 
the  possibility  of  combination  between  the  dust 
and  the  oxygen  or  gas  of  the  mine:  an  explosion 
may  occur  with  the  liberation  of  a  vast  quantity 
of  destructive  energy — heat,  light  and  force. 


HOW  ANIMALS  BREATHE  89 

The  oxygen  is  not  in  itself  a  source  of  this  energy, 
but  when  combined  with  carbon  it  gives  rise  to 
compounds  which  contain  energy,  and  when 
conditions  occur  under  which  the  energy  is  set 
free,  the  potential  energy  of  combination  becomes 
the  actual  energy  of  explosion.  The  air  of  the 
mine  becomes  charged  with  carbon  dioxide, 
carbon  monoxide,  or  both. 

There  is  a  sense  in  which  all  muscular  con- 
tractions are  explosions.  The  carbon  of  the 
food,  disseminated  through  the  body  by  the 
blood,  is  carried  to  the  muscles  for  the  repair 
of  their  material.  The  oxygen  of  the  lungs  is 
also  disseminated  by  the  same  blood  to  the 
muscles.  Here,  in  accordance  with  the  banking 
practice  of  the  body,  it  is  stored  in  reserve,  and 
from  this  reserve  there  is  given  out,  every  time 
a  contraction  or  movement  occurs,  a  cheque 
of  oxygen.  These  combining  with  the  muscle- 
matter  cause  an  explosion  or  series  of  explosions 
which  are  usually  inaudible.  Under  some  great 
mental  disturbance,  however,  a  contraction  of 
unusual  dimensions  and  energy  may  be  accom- 
plished. The  bank  may  be  broken.  A  vast 
output  of  oxygen  suddenly  combines  with  whole 
regiments  of  muscle-columns  and  the  muscle  may 
be  heard  to  crack.  Such  an  explosion  may  save 
a  man's  life,  but  it  not  uncommonly  comes  near 
to  exhausting  him  fatally. 


90  THE  ANIMAL  WORLD 

ANAEROBIC  ANIMALS. — The  primitive  breath- 
ing mode  lies  in  obtaining  means  for  the  energy- 
production  in  the  body  by  liberating  energy 
from  the  food.  All  animal-foods  contain  car- 
bon, hydrogen  and  oxygen;  proteid  foods  con- 
tain nitrogen  and  many  other  substances.  The 
splitting  of  the  food-compound  is  effected  by 
a  ferment;  that  is  to  say,  a  substance  capable 
of  producing  far-reaching  decomposition  without 
itself  undergoing  any  readily  perceptible  alter- 
ation. Thus  diastase,  one  of  the  commonest 
ferments  in  plants,  converts  starch  into  sugar. 
By  this  means  seeds,  spores,  many  bacteria, 
many  protozoa,  eel-worms,  even  leeches,  are 
able  to  obtain  an  amount  of  energy  that  suffices 
for  certain  manifestations.  Such  beings  are 
called  anaerobic,  as  they  can  subsist  and  move 
without  a  supply  of  atmospheric  oxygen. 

AEROBIC  ANIMALS. — Speaking  broadly,  how- 
ever, animals  and  plants  need  free  oxygen  and 
cannot  work  merely  by  the  decomposition  of  a 
complex  solid.  Whether  in  other  planets  beings 
can  utilize  oxygen  in  some  extracted  form  or 
substitute  some  other  element  for  their  energetics, 
we  know  not,  but  it  is  at  least  singular  that  oxygen 
is  so  plentiful  in  our  atmosphere  and  oceans. 
Without  an  oxygen-containing  atmosphere  so 
plentiful  as  our  own,  there  might  still  be  life  so 
long  as  some  oxygen,  carbon  and  nitrogen  were 


HOW  ANIMALS  BREATHE  91 

present.  Probably,  however,  the  abundance 
and  high  development  of  life  on  the  earth  is 
related  directly  to  the  great  stores  of  atmosphere 
in  the  rivers  and  seas,  in  the  porous  soil,  caverns 
and  subterranean  waters  as  well  as  in  the  en- 
velope of  the  globe.  These  various  atmospheres 
are  not  alike;  they  owe  their  different  compo- 
sitions in  part  to  gases  exhaled  by  the  earth, 
in  part  to  varying  temperature  and  pressure, 
and  in  part  to  the  degree  of  diffusion  of  the 
"normal"  atmosphere  into  the  depths  of  water 
and  of  earth. 

This  atmosphere  is  to  most  life  highly  nutri- 
tious, at  least  in  oxygen,  but  the  good  in  it  is 
hard  to  assimilate.  Air  is  a  mixture  of  (from 
one  point)  good,  bad  and  indifferent  things — 
oxygen,  carbonic  acid  and  nitrogen,  together 
with  a  vast  quantity  of  water-vapour,  dust  and 
spores.  Water  contains  dissolved  air  with  all 
its  complications  added  to  those  of  salt,  chlorine 
and  other  elements  of  the  ocean.  The  problem 
of  life-energetics  is  how  to  extract  this  oxygen 
from  the  atmosphere  and  how  to  combine  it 
with  oxidizable  material,  in  order  to  furnish 
heat,  movement,  reserve  stores  of  energy,  and 
growth.  Here  is  a  complicated  chemical  prob- 
lem which  we  solve  twenty  times  a  minute  with- 
out consciously  performing  anything:  only  when 
running  or  walking  at  high  altitudes  are  we  aware 


92  THE  ANIMAL  WORLD 

of  an  effort,  and  even  that  is  merely  the  effort 
of  inhaling  air.  We  are  absolutely  unaware 
of  the  means  by  which  the  air-mixture  is  partly 
robbed  of  its  oxygen,  still  less  of  the  way  in  which 
the  extracted  oxygen  is  disseminated,  combined, 
exploded  and  stored.  We  know  that  the  atmos- 
phere dissolved  in  water  can  only  be  made  to 
yield  up  its  oxygen  to  a  chemist  by  treatment, 
but  a  fish,  insect-larva,  snail  or  coral  can  extract 
it  without  diminishing  the  pressure  or  altering 
the  temperature  of  the  water.  Moreover,  the 
breathing  actions  of  a  man  or  of  an  animal  are 
not  uniform  and  mechanical  in  a  simple  sense: 
but  are  adapted  to  the  varied  needs  and  rhyth- 
mical phases  of  life.  Oxygen  enough  for  a  seden- 
tary life  is  not  sufficient  for  a  strenuous  one, 
and  the  respiratory  mechanism  is  plastic,  now 
meeting  the  demand  by  simple  diffusion  and 
now  by  secretion  or  by  drawing  on  the  reserve 
fund  in  case  of  a  run  on  the  bank  caused  by 
climbing,  gymnastics  or  a  race  for  life. 

THE  RESPIRATORY  MECHANISM. — The  res- 
piratory mechanism  consists  of  three  parts: 
(1)  The  means  whereby  oxygen  is  absorbed 
from  the  atmosphere,  (2)  the  means  by  which 
it  is  diffused  and  brought  to  the  various  tissues, 
and  (3)  the  respiratory  exchange  in  the  tissues 
leading  to  exhalation  of  carbonic  acid  and  water 
from  the  body.  The  term  "respiratory  organs" 


HOW  ANIMALS  BREATHE  93 

is  usually  confined  to  the  first  of  these  parts,  and 
in  this  restricted  sense  respiratory  organs  are 
either  gills,  that  is,  organs  for  absorbing  dis- 
solved atmospheric  oxygen,  or  lungs,  by  which 
are  meant  all  means  for  absorbing  oxygen  from 
the  atmosphere  directly. 

ENERGY  DERIVED  FROM  FOOD. — The  need 
for  oxygen  varies  greatly  in  different  animals 
and  even  in  the  same  animal  at  different  times 
and  seasons  of  its  life.  The  anaerobic  bacteria, 
certain  Protozoa,  eel-worms  and  leeches  can 
be  kept  in  an  atmosphere  deprived  of  oxygen 
for  a  long  time  without  diminution  of  their 
vital  processes,  since,  as  already  explained, 
these  animals  have  the  power  of  splitting  up 
the  reserve  food  in  their  bodies  and  making 
use  of  the  heat  so  liberated  by  the  "reducing" 
changes  that  give  the  necessary  energy  for  their 
movements  and  life-processes.  This  goes  on 
so  long  as  new  supplies  of  food  are  available  and 
on  condition  that  the  waste  materials,  such  as 
carbon  dioxide  and  waste  nitrogen,  are  removed. 
This  may  be  called  respiration  by  reducing 
ferments. 

RESPIRATION  OF  CCELENTERATES. — Above  Pro- 
tozoa we  have  first  the  sponges  and  Coelenterates 
(hydroids,  medusae,  sea-anemones  and  corals). 
With  regard  to  sponges  the  only  process  needing 
much  oxygen  is  that  of  growth,  and  it  is  probable, 


94  THE  ANIMAL  WORLD 

though  not  as  yet  established,  that  in  this  case 
there  is  a  ferment  capable  of  fixing  the  oxygen 
dissolved  in  the  sea,  or  in  fresh  water.  Currents 
of  water  are  almost  constantly  passing  through 
a  sponge  and  it  may  be  that  the  tissues  them- 
selves without  any  other  aid  are  able  both  to 
extract  the  dissolved  air  to  utilize  the  oxygen 
and  to  discharge  the  carbonic  acid  into  the  out- 
flowing current.  Such,  at  any  rate,  seems  to 
be  the  only  hypothesis  available  for  explaining 
the  respiration  of  Ccelenterates.  These  animals 
are  traversed  by  an  ingoing  and  an  outgoing 
stream,  or  by  an  irregular  flow  of  water  in  their 
central  cavity.  In  this  case,  again,  our  present 
knowledge  does  not  allow  of  a  confident  state- 
ment as  to  how  the  dissolved  oxygen  is  fixed. 
We  can,  however,  say  that  there  are  no  definite 
respiratory  organs  and  no  diffusing  medium 
until  we  reach  Ccelomic  animals.  In  Acoelomate 
creatures  there  is  either  selective  absorption  of 
oxygen  directly  from  the  atmosphere  dissolved 
in  the  water  or  an  oxydasic  ferment  in  the  tissues 
traversed  by  the  water.  The  larger  jelly-fish 
are  in  this  respect  the  most  needy  of  the  Coelen- 
terates  and  require  as  much  oxygen  per  unit  of 
body  weight  as  does  a  frog.  If  kept  in  incom- 
pletely aerated  water,  these  pelagic  medusse 
shrink  to  a  mere  fraction  of  their  former  size. 
Temperature  also  affects  the  demand  for  oxygen, 


HOW  ANIMALS  BREATHE  95 

for  as  the  sea  becomes  warmer  the  processes  of 
life,  up  to  a  point,  go  on  more  quickly  and  the 
demand  for  food  and  oxygen  becomes  more 
insistent. 

EVOLUTION  OF  GILLS.  MOLLUSCA.  —  In  Coelo- 
mate  animals  we  find  not  only  definite  respiratory 
organs  but  a  blood  system  for  diffusing  the  oxy- 
gen, for  carrying  it  to  the  tissues  and  for  dis- 
charging carbonic  dioxide  and  waste  matters 
from  them.  Beginning  with  the  Mollusca,  in 
which  movement  is  slow,  growth  slow  and  egg- 
production  large,  we  find  a  variety  of  gill-like 
structure  produced  at  different  points  or  along 
different  lines  in  the  several  classes.  These  gills 
are  feathery  or  plate-like  outgrowths  of  the  body- 
wall  and  are  placed  in  one  or  two  pairs  along  the 
side  of  the  body.  As  Mollusca,  unlike  the  Accelo- 
mata,  do  not  swallow  and  exhale  currents  of  water, 
their  gills  have  to  extract  the  oxygen  from  water 
that  flows  over,  instead  of  inside  them.  In  order 
to  facilitate  this  flow,  the  gills  are  commonly  en- 
closed in  a  tubular  fold  of  the  body-wall  so  ar- 
ranged as  to  permit  of  water  entering  at  one  end 
and  escaping  at  the  other.  This  fold  is  termed 
the  mantle  and  is  usually  encased  in  a  shelly 
secretion.  Hence  by  inspection  of  the  shell  only, 
a  good  deal  can  be  inferred  as  to  the  nature  of  the 
mollusc  that  it  surrounded.  The  cap-like  shells 
of  Patella  (the  limpet)  are  related  to  a  circu- 


96  THE  ANIMAL  WORLD 

lar  series  of  gill-plumes  that  almost  surround 
the  foot;  whereas  the  ormer- shell  with  its  row 
of  holes,  and  the  slit-like  opening  of  the  shell 
in  Fissurella  allow  a  definite  current  to  enter 
the  mantle  at  this  point,  to  bathe  the  pair  of 
feathery  gills  that  lie  at  the  sides  of  the  neck 
and  to  escape  at  the  sides  of  the  foot.  The 
twisted  shells  of  the  ordinary  marine  snails  have 
either  a  notched  or  a  smooth  lip.  If  notched 
the  shell  belongs  to  a  siphonate  carnivorous 
mollusc,  for  the  notch  lodges  a  special  tubular 
outgrowth  of  the  mantle  (the  siphon)  which  per- 
mits of  water  being  inhaled  by  such  snails  just  as 
an  elephant  draws  water  with  his  trunk;  but  un- 
like the  case  of  the  elephant  the  water  after  pass- 
ing through  the  siphon  bathes  the  pair  of  gills 
and  escapes  at  the  margin  of  the  foot.  If  the 
shell  be  smooth  lipped,  the  snail  inhabiting  it 
will  be  vegetarian  and  without  any  special 
means  of  water  supply.  The  tusk-shell  (Den- 
talium)  (Fig.  12)  is  perfectly  tubular  and  in  this 
animal  there  are  no  definite  gills. 

Amongst  bivalves  the  mantle  is  drawn  out  in 
short  or  long  siphons  containing  extensions  of 
the  mantle-cavity.  Water  is  inhaled  by  ciliary 
action  of  the  gills  (plate-like  folds  of  the  body- 
wall)  and  enters  the  lower  siphon,  passes  round 
and  between  the  gill-filaments  and  escapes  into 
the  upper  or  exhalent  siphon.  In  burrowing  bi- 


HOW   ANIMALS  BREATHE          97 


A. 


Fig.  12.  —  Three  of  the  oldest  animals,  persistent  types  as 


98  THE  ANIMAL  WORLD 

valves,  such  as  Venus,  the  ship  worm  (Teredo), 
the  clam  Mya,  and  many  others,  the  siphons  are 
greatly  elongated  to  enable  the  animals  to  bury 
themselves  and  tap  fresh  water  from  above  the 
surface  of  the  sand  or  mud.  These  long  siphons 
cannot  be  wholly  retracted  into  the  shell,  and  the 
space  they  occupy  leaves  an  indented  line  on  the 

they  are  often  called,  which  have  lived  on  generation 
after  generation  since  early  geological  time. 

A.  Dentalium:   the  tusk-shell  (actual  length  about  1  $  inches) 

with  the  animal  in  situ.  This  remarkable  mollusc  lives 
in  sand  round  our  coasts,  and  gathers  its  food  by  means 
of  peculiar  tentacles  (T)  ranged  round  the  mouth.  It 
burrows  by  the  aid  of  an  acorn-shaped  foot  (F).  The 
shell  (SH)  is  tubular,  the  upper  end  being  closed  by 
a  valve  (not  shown).  This  animal  has  persisted  from 
Devonian  times  to  the  present  day.  _ 

B.  Lingula:    the  lamp-shell,  still  found  living  on  the  coasts 

of  Japan,  the  Sandwich  Islands,  and  elsewhere,  is  a 
representative  of  the  class  Brachiopoda.  Though 
superficially  like  bivalve  molluscs  in  possessing  shells, 
these  animals  are  really  very  isolated  from  all  others. 
Many  of  them  were  formerly  so  numerous  as  to  make 
up  the  greater  part  of  limestone  ranges.  To-day  they 
are  dying  out.  They  are  the  oldest  of  all  animals  at 
present  known,  and  some  such  as  Lingula  have  per- 
sisted unchanged  from  early  Cambrian  times,  that  is 
to  say,  the  earliest  fossiliferous  strata. 

Lingula  consists  of  two  valves  (V,  V),  and  lies  ver- 
tically in  the  sand  with  the  stalk  (St)  buried.  (  X  $.) 

C.  Nautilus:    represented   as   cut  vertically.     This  ancient 

animal  also  goes  back  to  Cambrian  times,  and  is  the 
sole  living  representative  of  an  order  of  Cephalopoda 
characterized  by  having  four  gills,  four  kidneys,  and 
an  external  shell.  It  lives  in  deep  water  in  the  seas 
round  New  Britain  and  the  Malay  Peninsula.  Its 
congeners  were  the  Ammonites  (or  Ram's  horns),  so 
commonly  seen  as  cottage  ornaments  in  districts  where 
the  liassic  rocks  come  to  the  surface. 

The  term  "Cephalopod"  is  due  to  the  fusion  of  the 
foot  with  the  head  (F).  The  eye  (E)  is  inserted,  the 
breathing  siphon  (S)  and  two  gills  (G,  G)  are  also  shown. 
(i  natural  size.) 


HOW  ANIMALS  BREATHE  99 

inner  surface  of  the  shell.  Thus  the  habit  and 
shape  of  the  animal  can  to  some  extent  be  in- 
ferred from  an  examination  of  the  dried  shell. 

LUNGS  OF  MOLLUSCA. — The  most  interesting 
modification  of  these  molluscan  breathing  organs 
occurs  in  the  terrestrial  snails  and  slugs.  Nearly 
all  phyla  of  Coelomates  have  managed  to  estab- 
lish an  outpost,  as  it  were,  on  land;  many  in- 
deed have  wholly  relinquished  the  water  for  the 
land.  In  all  such  migrations  and  colonies,  the 
breathing  organs  have  to  be  changed  from  gills 
to  lungs.  In  a  damp,  tropical  atmosphere,  gill- 
breathing  might  still  be  carried  out  on  land  for  a 
short  time,  but  if  the  colonization  is  to  be  exten- 
sive and  cosmopolitan,  lungs  become  a  necessity. 
Such  a  change  of  function  involves  much  change 
of  structure.  A  gill  is  a  delicate  outgrowth 
totally  unsuitable  to  resist  desiccation.  A  lung 
must  be  capable  of  expanding  and  of  contracting, 
and  at  the  same  time  of  remaining  unaffected  by 
the  double  draught  of  air  that  passes  through  it. 
Land  life  demands  increased  muscular  efficiency, 
as  we  have  seen,  not  only  to  support  weight,  but 
to  push  the  weight  and  to  overcome  the  friction 
of  the  rocks  and  soil:  and  now,  with  breathing, 
there  comes  into  the  problem  a  further  muscular 
demand,  namely,  that  for  expanding  the  lung 
and  so  drawing  in  air,  and  for  contracting  it  and 
so  expelling  air.  The  lung  itself  is  simply  the 


100  THE  ANIMAL  WORLD 

mantle-cavity — the  space  in  which  the  gills  used 
to  be.  The  walls,  even  in  aquatic  molluscs,  are 
vascular,  but  they  are  not  rhythmically  con- 
tractile. Tied  as  they  are  to  the  shell  they  cannot 
move.  The  only  thing  that  can  move  is  the  foot — 
the  ventral  muscular  part  of  a  Mollusc's  body. 
Here,  then,  is  the  means  of  altering  the  lung's 
capacity.  The  foot  must  be  made  to  elongate 
and  flatten  out,  so  expanding  the  cavity;  and  to 
contract  and  arch,  so  diminishing  it.  The  open- 
ing of  the  mantle-cavity  must  be  narrowed  to  a 
slit  and  then  the  lung  can  be  made  effective. 

RESPIRATORY  PIGMENTS.  —  The  second  prob- 
lem, how  to  extract  and  diffuse  the  oxygen  from 
water-air  or  atmospheric-air  must  be  noticed. 
Two  methods  have  been  adopted:  the  use  of  res- 
piratory pigments  and  the  use  of  oxidasic  fer- 
ments. Respiratory  pigments  are  substances 
that  have  an  affinity  for  oxygen,  somewhat,  for 
instance,  as  sodium  has.  They  have  the  power 
of  combining  loosely  with  atmospheric  oxygen, 
so  that  it  can  be  split  off  from  the  pigment  with- 
out chemical  decomposition.  Two  such  pigments 
are  known  and  both  occur  in  Molluscs.  The 
first — haemoglobin — confers  the  colour  upon  red 
blood;  the  second — hsemocyanin — gives  blood 
a  blue  colour  when  oxidized.  Red  blood  occurs  in 
very  few  Molluscs,  as  indeed  the  restricted  calls 
upon  their  oxygen-capacity  would  lead  one  to 


HOW  ANIMALS  MEATlIE       l61 

expect.  Hsemocyanin  is  more  commonly  present. 
This  pigment  is  not  unlike  haemoglobin,  but  con- 
tains copper  in  place  of  iron  and  has  one-fourth 
its  oxygen-holding  capacity. 

RESPIRATORY  FERMENTS. — The  second  method 
of  oxygenating  the  tissues  lies  in  the  presence  of 
oxidasic  ferments.  In  some  cases  (Mussels)  these 
ferments  which  fix  oxygen  are  attached  to  pig- 
ments, in  others  they  are  uncoloured.  One  pecu- 
liarity of  Mollusca  must  be  noticed,  which  is  that 
of  a  means  of  banking  a  reserve  of  gases  upon 
which  the  body  can  draw  at  times  of  scarcity.  In 
the  highest  Molluscs — nautilus,  cuttlefish  and 
their  allies — the  process  of  respiration  is  forcible, 
corresponding  to  the  great  muscular  exertion  and 
great  size  of  these  creatures.  Moreover,  some 
of  them  descend  to  great  depths  and  in  doing 
so  experience  rapid  changes  of  pressure,  which 
are  reversed  on  rising  again  towards  the  surface. 
The  blood  tends  at  such  times  to  part  with  its 
contained  gases,  and  if  the  gas  can  be  collected 
its  oxygen  might  be  available  for  subsequent 
contraction.  Nautilus  has  a  vascular  gland  by 
which  the  gases  are  exhaled  and  collect  in  the 
shell,  but,  so  far  as  is  known,  their  use  is  rather 
hydrostatic  than  respiratory  (Fig.  1£). 

RESPIRATION  OF  ANNELIDS. — The  two  great 
phyla  of  animals  with  segmented  appendages — 
the  Annelids  and  the  Arthropods — have  means  of 


102  THE  ANIMAL  WORLD 

respiration  that  vary  according  to  their  habitat. 
In  aquatic  Annelids  the  blood  is  either  red  or 
green,  the  green  colour  being  due  to  a  pigment  that 
is  very  rarely  met  with  in  animals.  The  body- 
wall  is  drawn  out  into  the  form  of  tufts  or  exter- 
nal gills,  a  pair  of  which  occur  on  each  segment  or 
a  cluster  may  be  developed  at  the  head-end  or 
the  tail-end.  The  tubicolous  Annelids  have  a 
superb  coronal,  a  spiral  outgrowth  bordered  by 
cilia,  down  which  a  current  of  water  is  constantly 
proceeding  to  the  mouth,  aerating  the  branchial 
coronal  on  its  way,  and  combining,  as  is  fre- 
quently the  case,  the  function  of  respiration 
with  that  of  alimentation.  In  many  of  the  fresh- 
water Annelids  a  current  of  water  is  drawn  into 
the  food  canal  through  the  aboral  opening 
and  probably  serves  as  an  accessory  respiratory 
stream .  In  earth-worms  the  highly  vascular  body- 
wall,  kept  constantly  moist  by  its  own  mucous 
investment  and  by  contact  with  damp  soil  and 
air,  is  the  most  efficient  means  of  extracting  oxygen 
from  the  air  around  it.  In  all  these  Annelids  the 
red  blood  has  the  same  pigment  as  our  own  and 
has  similar  properties  of  combining  with  oxy- 
gen, yielding  it  to  the  tissues  and  of  returning  to 
the  skin  laden  with  carbon  dioxide. 

AIR-BREATHING  ARTHROPODS. — Among  Arthro- 
pods two  very  distinct  types  of  respiratory  organs 
are  found  among  the  terrestrial  and  the  aquatic 


HOW  ANIMALS  BREATHE         103 

orders  respectively.  The  first  are  known  as  tra- 
cheae and  consist  of  branched  tubes  starting  from 
a  common  hollow  stem  which,  in  turn,  opens  to 
the  exterior  through  the  body-wall.  These  tra- 
cheae occur  in  pairs  and  are  present  in  most  of 
the  body  segments.  The  openings  or  "spiracles" 
are  provided  with  muscles  and  the  whole  mecha- 
nism is  adapted  for  the  inspiration  of  air  and  the 
expiration  of  carbonic  acid.  The  body  of  an  in- 
sect readily  shows  rhythmical  breathing  move- 
ments by  which  the  tracheae  are  alternately  com- 
pressed and  expanded,  but  the  branches  of  the 
system  are  so  extremely  fine  that  mere  muscular 
pressure  will  not  explain  how  the  air  moves  along 
them,  and  the  mechanism  of  respiration  by  means 
of  tracheae  is  still  far  from  being  understood.  At 
intervals  the  tracheae  are  (especially  in  winged 
insects)  enormously  enlarged  to  form  air-sacs 
and  air-cushions.  These  110  doubt  lighten  the 
body,  but  they  probably  serve  another  purpose, 
namely,  to  provide  a  reservoir  of  air  from  which 
the  fine  branches  are  filled  by  diffusion  and  into 
which  the  carbon  dioxide  is  discharged. 

There  is  one  great  characteristic  of  the  mode 
of  breathing  in  tracheate  Arthropoda  (that  is, 
the  sub-division  including  Myriapoda,  Insects, 
Spiders  and  their  allies,  and  the  strange  archaic 
Peripatus).  It  consists  in  the  diffusion  of  air  by 
ramifications  of  the  tracheae  instead  of  by  blood- 


104  THE  ANIMAL  WORLD 

vessels.  The  blood  system  is  poorly  developed 
and  probably  serves  merely  for  the  absorption  and 
diffusion  of  the  food.  In  a  few  insect-larvse,  how- 
ever (such  as  the  "blood-worm"  or  larva  of  the 
gnat  Chironomus),  red  blood  is  present  and  serves 
to  extract  oxygen  from  the  stagnant  water  in 
which  these  gnats  pass  their  earlier  stages  of  de- 
velopment. In  no  adult  insect,  however,  is  there 
a  circulation  of  oxygen  by  the  blood.  This  inva- 
sion of  the  tissues  by  multitudinous  branching 
tubes  permits  of  a  thorough  aeration;  provides 
energy  for  sustained  muscular  efforts;  supplies 
(through  the  consequent  oxidations  of  the  tissues) 
a  certain  advantageous  degree  of  heat  above  that 
of  the  surrounding  air  (as  in  bees);  enables  re- 
serve material  to  absorb  energy  for  the  produc- 
tion of  eggs  in  successive  clutches;  and  also  to 
some  extent  provides  a  means  for  exhaling  the 
carbon  dioxide  produced  by  oxidation.  It  is  prob- 
able that  the  larval  histories  of  insects  will  yield 
many  interesting  additional  facts  to  the  known 
means  of  respiration,  for  many  flies  which  require 
an  ample  supply  of  atmosphere  when  winged, 
pass  their  larval  life  in  surroundings  that  are  al- 
most without  oxygen;  for  example,  as  parasites 
in  the  stomach  of  the  horse  (Gastrophila  Equi),  in 
wood  of  trees  and  the  fleshy  substance  of  nuts, 
galls,  etc.  Probably  in  these  cases  some  ferment  of 
a  reducing  nature  is  present.  An  especially  inter- 


HOW  ANIMALS  BREATHE         105 

esting  modification  of  insect-structure  is  seen  in 
the  telescopic  tail  of  the  rat-tailed  larva  of  Eris- 
taliSy  the  drone-fly.  This  fly,  so  bee-like  as  to 
give  rise  to  the  legend  of  bees  arising  from  oxen 
or  lions  (in  Samson's  riddle  "Out  of  the  eater 
came  forth  meat  and  out  of  the  strong  came  forth 
sweetness"),  lays  its  eggs  in  shallow  pools  of 
water  or  of  putrescent  matter.  The  larvae  hatched 
from  these  eggs  have  a  tracheal  system  which 
opens  by  a  tail-tube  to  the  atmosphere.  This 
tube  is  telescopic  and  can  be  extended  to  a  dis- 
tance of  several  inches.  By  this  means  the  larvse 
ensure  a  supply  of  air  in  case  of  rain  or  of  drought. 
GILL-BREATHING  ARTHROPODS. — Turning  now 
to  the  branchiate  Arthropods  we  find  Crustacea 
breathing  almost  entirely  by  plate-like  or  plu- 
mose outgrowths  of  the  legs.  In  the  lower  Crus- 
tacea these  legs  are  all  or  nearly  all  alike  and  are 
paddle-like  structures,  similar  in  form  to  a  pal- 
mate leaf,  and  may  number  seven  pairs  or  seventy. 
In  higher  Crustacea  the  plate-like  gills  are  con- 
fined to  the  limbs  of  the  middle  body-section  or 
thorax.  In  all  cases,  however,  the  gills  consist 
essentially  of  one  or  more  lobes  placed  on  the 
inner  side  of  the  swimming  or  walking  limbs. 
Blood,  propelled  by  the  heart,  wanders  into 
spaces  in  these  lobes  and  here  lies  separated  from 
the  surrounding  water  by  a  mere  film  of  tissue. 
This  blood  is  colourless,  not  red,  green  or  blue,  as 


106 


THE  ANIMAL  WORLD 


in  Annelids  and  Molluscs.     Sometimes  a  faint 
blue  tinge  is  observable,  and  it  is  known  that 


Fig  13. — Group  of  Crustacea. 

A.  Fairy  Shrimp  (Chirocephalus  diaphanus) :    found  in  a  few 

isolated  fresh-water  ponds  in  England,  and  more  abun- 
dantly in  Europe,  Asia,  and  America.  The  animal 
lies  on  its  back  on  the  surface  of  the  water,  and  rows 
itself  along  by  paddling  movements  of  the  limbs.  These 
movements  produce  a  wave  of  motion  passing  from 
before  backwards. 

This  crustacean  belongs  to  the  group  Phyllopoda,  so 
called  from  the  leaf -like  character  of  the  feet.  (  X  1.) 

B.  Daphnia:    a  common  fresh- water  shrimp  or  water  flea; 

also  belonging  to  the  Phyllopoda.  The  body  is  enclosed 
in  a  shell,  from  which  the  two  great  antennae,  or  rowing 
organs,  project  in  front  over  the  compound  eye.  (X  30.) 

C.  Pandalus:    one  of  the  common  marine  prawns  belonging 

to  the  higher  group  of  Crustacea — the  Carida,  or  true 
shrimps.  (  X  1.) 


HOW  ANIMALS  BREATHE         107 

many  Crustacea  have  the  same  copper-containing 
substance — Hsemocyanin — that  occurs  in  cuttle- 
fish. Either  by  the  combining  power  of  this  pig- 
ment or  possibly  by  some  more  subtle  process  of 
absorption,  oxygen  is  extracted  from  the  water 
and  taken  into  the  blood;  at  the  same  time  some 
carbonic  acid  is  probably  discharged. 

In  most  Crustacea  the  gills,  like  those  of  Mol- 
luscs, are  enclosed  in  folds  of  the  body-wall  in 
order  to  ensure  a  more  definite  current  of  water 
and  more  thorough  aeration.  This  device  has  a 
nutritive  value  also,  as  the  lower  members  of  the 
class,  such  as  Daphnia,  are  thereby  enabled  to 
sweep  particles  of  food  into  the  mouth.  These 
folds,  or  carapace  as  they  are  collectively  termed, 
may  box  in  the  body  just  as  a  bivalve  shell  encloses 
a  mussel.  Such  little  bivalve  Crustacea  are  ex- 
tremely common  and  constitute  one  of  the  most 
efficient  scavenging  class  of  aquatic  animals. 

In  the  larger  and  more  complex  Crustacea, 
simple  casual  movements  of  the  limbs  are  not 
enough  to  ensure  a  sufficient  supply  of  oxygen 
for  the  gills;  and  to  amend  this,  the  gills  are 
moved  to  the  base  of  the  leg  or  on  to  the  body 
and  are  then  bathed  by  a  constant  respiratory 
current  which  is  carried  out  by  the  rhythmical 
and  unceasing  vibration  of  one  of  the  anterior 
limbs  (the  second  maxillse).  The  gill  chamber  is 
so  arranged  as  to  communicate  by  two  narrow 


108  THE  ANIMAL  WORLD 

openings,  and  as  fast  as  the  water  is  baled  out  by 
the  anterior  slit,  a  fresh  supply  is  inhaled  through 
the  posterior  one  which  lies  behind  the  last  leg  in 
prawns  and  in  front  of  the  first  in  crabs.  The 
modifications  of  this  arrangement  are  of  great  in- 
terest. Burrowing  crabs,  sunk  in  the  sea-sand, 


Fig.  14. — Anterior  half  of  the  body  of  a  Crayfish  seen  from 
the  left  side,  with  the  carapace  or  body  shield  removed 
to  show  the  gills.  The  legs  have  been  cut  off  short. 
(X2.) 

The  gills  are  seen  lying  one  over  another,  and  in  front 
of  the  first  gill  is  the  baler  (B),  which  in  life  is  con- 
stantly baling  water  out  through  the  aperture  just  above 
the  reference  line.  Fresh  water  flows  in  at  C.  The 
chamber  in  which  the  gills  are  lying  is  a  space  enclosed 
by  a  fold  of  the  body-wall,  and  not  actually  inside  the 
body. 

would  obviously  soon  block  their  inhalent  open- 
ing with  grit  if  they  employed  this  method  exclu- 
sively. Hence  Corystes,  the  masked  crab,  for 
example,  apposes  the  first  pair  of  long,  hairy 
antennse,  thus  forming  a  tube  long  enough  to 
reach  up  to  the  top  of  the  burrow.  The  baling 
appendage  now  reverses  its  usual  action  and 


HOW  ANIMALS  BREATHE         109 

draws  water  free  from  sand  down  the  antennal 
tube  and  into  the  anterior  slit.  When,  however, 
Corystes  leaves  its  burrow  it  can  employ  the  nor- 
mal respiratory  current.  In  some  prawns  and 
crabs  this  reversal  of  the  direction  of  the  current 
is  a  usual  occurrence,  but  its  significance  is  not 
fully  appreciated. 

LAND-CRABS. — An  interesting  modification  of 
the  breathing  apparatus  occurs  in  some  land-crabs. 
In  warm  moist  climates  and  even  along  the 
Mediterranean,  many  crabs  have  betaken  them- 
selves to  a  terrestrial  life,  some  living  just  above 
high-water  mark,  others  right  inland,  and  some 
modified  hermit-crabs  have  even  the  power 
of  climbing  trees  in  search  of  cocoanuts.  In 
most  of  these  crabs  the  gills  are  retained,  and 
sufficient  moisture  is  left  in  the  tightly-closed 
gill-chambers  to  prevent  the  desiccation  of 
these  delicate  organs;  such  crabs,  however,  re- 
pair periodically  to  the  sea  to  renew  their  supply 
of  water  or  to  spawn.  Some  others,  notably  the 
coco-  or  robber-crab  (Birgus),  whilst  retaining 
the  gills  in  a  reduced  form  have  converted  the 
greater  part  of  the  gill-chamber  into  a  lung, 
the  walls  of  which  are  vascular  and  enable  the 
robber-crab  to  breathe  air  directly,  and  so  to 
remain  for  months  away  from  water;  in  fact, 
only  the  breeding  season  necessitates  a  return. 

RESPIRATION    IN  VERTEBRATES. — Passing    on 


110  THE  ANIMAL  WORLD 

now  to  the  vertebrate  animals,  we  have  the 
broad  distinction  between  fish  with  gills  and 
other  vertebrates  with  lungs.  The  sharpness 
of  the  distinction  is,  however,  lessened  by  the 
peculiar  breathing  organs  of  the  Amphibia, 
and  of  certain  fish.  Gill-breathing,  neverthe- 
less, is  absolutely  unknown  in  reptiles,  birds 
and  mammals  at  any  stage  or  era.  This  is 
one  of  the  many  indications  that  show  how 
marked  is  the  change  between  fish  and  am- 
phibia on  the  one  hand,  and  the  three  higher 
groups  on  the  other. 

GILLS  OF  FISH. — The  gills  of  the  fish  are 
composed  of  bright  red  tassels  set  on  hoops 
that  encircle  the  throat,  and  are  usually  cov- 
ered by  a  movable  flap — the  gill-cover.  Under 
this  flap,  the  neck  of  the  fish  is  perforated  by 
a  certain  number  of  crescentic  slits.  Between 
the  slits,  the  substance  of  the  neck  is  strength- 
ened by  gill-arches,  girdles  of  gristle  or  of  bone 
to  which  muscles  are  attached  so  that  the  gill- 
slits  may  be  opened  or  closed  at  will.  In  all 
cartilaginous  fish  (sharks,  rays,  dogfish)  the 
slits  open,  independently,  to  the  exterior  and 
are  not  covered  by  a  gill-flap.  In  bony  fish 
the  gill  cover  is  present,  and  its  rise  and  fall  is 
regulated  by  special  nerves  and  muscles.  In 
both  cases  the  blood-vessels  that  encircle  the 
neck  break  up  into  tufts  on  the  surface  of  the 


HOW  ANIMALS  BREATHE          111 

arches,  so  that  each  slit  is  bordered  by  two 
such  series  of  tufts. 

The  gill-apparatus  is  set  in  action  by  the 
following  mechanism.  The  throat  of  the  fish 
is  distended  by  the  contraction  of  the  muscles 
that  start  from  the  stiff  ventral  ends  of  the 
gill-arches,  and  as  the  mouth  opens  whilst  the 
gill-cover  is  kept  shut,  a  gulp  of  water  is  taken 
in  ;  the  mouth  is  then  closed,  the  throat  muscles 
rise,  diminishing  the  size  of  the  branchial  cavity 
and  so  force  the  water  out  through  the  gill- 
slits  and  under  the  gill-cover.  Thus  there  is 
a  rhythmical  inhaling  of  water  which  passes 
through  the  mouth,  over  the  gills  and  out  behind 
them.  During  its  passage  over  the  thin  vas- 
cular tassels,  the  oxygen  of  the  dissolved  air 
is  absorbed  by  the  haemoglobin  of  the  red  blood 
and  is  carried  away  to  supply  the  brain,  muscles, 
and  body  generally  :  at  the  same  time  carbon 
dioxide  is  exhaled  with  the  outgoing  stream. 

As  in  the  case  of  all  animal  adaptations, 
breathing  in  such  a  large  class  as  fishes  is  no 
mere  mechanical  and  uniform  arrangement, 
and  the  above  scheme  is  modified  to  adapt 
various  orders  for  their  varied  modes  of  life, 
and  is  even  altered  at  different  epochs  of  the 
life  of  an  individual.  For  example,  a  skate 
rests  on  the  sea-bottom  and  cannot  inhale 
water  through  its  mouth  without  swallowing 


THE  ANIMAL  WORLD 

also  quantities  of  debris,  hence  it  uses  a  pair 
of  large,  rhythmically-contractile  openings  which 
lie  on  its  upper  surface  just  behind  the  eyes. 
Through  these  "spiracles"  the  water  opens 
and  passes  over  the  gills.  Even  the  escapement 
action  of  the  gill-cover  is  nicely  adapted  for 
different  orders  of  fish.  In  pelagic  orders  the 
current  is  directed  backwards  so  as  to  aid  the 
forward  dart  of  these  active  animals — very  much, 
but  in  reversed  direction,  as  the  puff  of  the 
exhalent  current  sends  a  cuttlefish  shooting 
backwards.  Sedentary  fish,  on  the  other  hand, 
discharge  their  gill-water  downwards  so  as  to 
lift  them  slightly  ;  and  any  one  who  has  ex- 
amined fish  in  captivity  will  notice  the  variable 
extent  of  the  breathing  movements.  In  winter, 
dogfish,  and  probably  others,  breathe  at  half 
the  summer  rate  ;  sticklebacks  when  mating 
breathe  very  rapidly. 

AIR-BREATHING  FISH. — Perhaps  the  most  in- 
teresting feature  of  respiration  in  fish  is  the 
adaptation  occurring  in  several  unrelated  fam- 
ilies to  permit  of  breathing  air  directly  or  to 
allow  of  migrations  over  land.  The  common 
loach  sucks  down  air  as  well  as  water,  and  many 
North  American  fish,  the  gar-pike,  the  bowfin, 
as  well  as  the  Polypterus  of  the  upper  Nile  and 
other  African  rivers,  constantly  rise  to  the  sur- 
face, not  only  to  snatch  food,  but  to  inhale  air 


HOW  ANIMALS  BREATHE        113 

which  passes  down  their  throats.  In  the  family 
of  Siluroids,  to  which  the  cat-fish  belong,  there 
are  many  genera  which  possess  special  cham- 
bers, in  which  air  so  inhaled  can  be  stored,  and 
in  the  wholly  unrelated  climbing  perch  (Andbas) 
of  the  East  there  is  a  complicated  folding  of 
the  gill-cavity.  Whether  this  additional  supply 
of  air,  over  and  above  that  dissolved  in  the  water 
that  flows  round  the  gills,  is  needed  to  supple- 
ment the  lack  of  oxygen  in  the  water,  is  a  doubt- 
ful point.  The  interest  of  the  habit  lies  in  its 
bearing  on  the  way  in  which  animals  came  to 
leave  water  and  to  breathe  air. 

We  have  seen  that  such  emancipated  Molluscs 
and  Crustacea  converted  their  branchial  cav- 
ities into  a  lung,  and  there  are  many  indications 
of  the  same  change  of  habit  and  function  in 
fish.  A  common  Egyptian  siluroid,  for  ex- 
ample, leaves  the  lake  Kurun  (which  lies  to 
the  west  of  the  Nile,  on  the  borders  of  the 
Libyan  desert)  at  night  and  travels  over  the 
damp  grass,  returning  before  morning  from 
its  nocturnal  excursion.  The  springing  goby, 
Periophthalmus,  so  common  on  the  mangrove 
swamps  of  East  Africa  and  Malay,  never  becomes 
wholly  submerged,  but  roams  over  the  muddy 
flats  above  the  sea-margin,  keeping  its  tail  in 
water  when  at  rest.  The  climbing  perch  just 
referred  to  walks  up  trees  by  the  aid  of  its  ven- 


114  THE  ANIMAL  WORLD 

tral  spines,  and  is  enabled  to  do  so  by  providing 
itself  with  a  supply  of  water,  or  air  and  water, 
in  its  peculiar  pharyngeal  fold.  Young  eels 
(elvers)  annually  make  their  way  from  the 
rivers  overland  into  tracts  of  water  not  directly 
communicating  with  the  sea.  But  no  fish  that 
we  are  aware  of  has  adapted  itself  to  terrestrial 
life  in  a  permanent  fashion. 

THE  AIR-  OR  SWIM-BLADDER. — We  must 
now  briefly  consider  the  air-bladder,  which  is 
related  in  an  important  way  to  respiration. 
With  a  few  exceptions  (of  which  the  gristly  fish, 
flat  fish,  and  the  common  mackerel  are  the 
most  remarkable)  all  fish  possess  a  bladder 
under  the  backbone.  In  a  herring,  for  example, 
the  air-bladder  appears  as  a  silvery  tube  ex- 
tending from  the  gills  in  front  to  the  hinder 
part  of  the  body-cavity.  Careful  examination 
shows  that  it  opens  to  the  stomach  by  a  duct 
of  considerable  thickness,  which  perforates  the 
bladder  at  about  the  middle  of  its  length.  Such 
a  fish  is  able  to  pass  air  into  its  bladder  or  to 
allow  of  the  escape  of  the  gases  contained  in 
it.  The  surprising  fact,  however,  about  the 
air-bladder,  is  that  it  does  not  contain  air.  The 
gas  in  the  case  of  marine  fish  contains  more 
oxygen  with  traces  of  carbon  and  less  nitrogen. 
The  air-bladder  is  a  gas-cylinder  into  which 
the  oxygen  is  secreted  from  certain  arterial 


HOW  ANIMALS  BREATHE         115 

blood-vessels,  and  may  possibly  be  carried 
away  by  certain  vessels  for  the  aeration  of  the 
body.  A  certain  amount  of  air,  gulped  in  by 
the  fish,  does  probably  enter  the  bladder  by 
the  pneumatic  duct  or  passage  from  the  food- 
canal,  but  this  air  only  serves  to  dilute  the 
oxygen  which  is  extracted  from  the  arteries 
of  the  bladder.  In  all  fish  the  air-duct  is  present 
in  early  life,  as  it  is  the  stalk  of  the  process 
whereby  the  bladder  has  grown  out  of  the  ali- 
mentary canal;  but  in  many,  perhaps  in  the 
majority,  the  duct  closes  up  and  disappears. 
In  such  cases  the  gases  of  the  bladder  enter 
and  leave  it  exclusively  by  the  blood-vessels. 

From  this  point  of  view,  therefore,  the  air- 
bladder  is  another  example  of  the  banking  or 
reserve  principle  so  abundantly  illustrated  in 
animal  life.  It  represents  a  reserve  of  oxygen 
upon  which  the  body  may  draw  for  its  special 
needs.  Just  as  each  animal  banks  its  reserve 
of  food  in  some  chamber  (either  as  fat  or  starchy 
substances  in  the  skin  and  liver),  and  only 
utilizes  a  working  supply,  or,  again,  as  animals 
employ  only  a  portion  of  the  oxygen  stored 
up  in  their  muscles,  so  it  seems  that  fish  only 
abstract  small  quantities  of  their  central  store 
of  oxygen. 

RESPIRATORY  PIGMENTS  IN  FISH.  —  The 
principle,  however,  goes  further  than  this,  for 


116  THE  ANIMAL  WORLD 

most  powerful  and  pelagic  fish  have  still  another 
means  of  storing  oxygen  and  of  rendering  their 
muscles  capable  of  prolonged  and  untiring 
work.  The  red  colour  of  salmon,  the  dark 
brown  colour  of  mackerel,  and  the  dark  red 
tint  of  tunny  are  due  to  a  form  of  haemoglobin 
stored  in  the  muscle  itself.  Such  fish  are  emi- 
nently free  and  far  wandering.  Unlike  the 
flat-fish,  which  rest  long  hours  in  one  position, 
unlike  even  the  cod  and  haddock  which,  though 
constantly  moving,  are  not  great  travellers, 
the  dark-fleshed  pelagic  fish  pursue  their  prey 
actively  and  migrate,  in  some  cases,  far  up 
stream  to  spawn,  and  down  to  the  deep  sea 
to  feed.  Dark  muscular  colour  is  a  sign  of 
greater  powers  of  endurance,  of  exertion,  and 
of  production,  and  is  seen  not  only  in  pelagic 
fish,  but  in  the  most  strenuous  muscles  of  Mol- 
luscs, of  Annelids,  of  insects  and  birds  and 
mammals. 

RESPIRATION  IN  AMPHIBIA. — With  regard  to 
the  breathing  of  Amphibia  a  few  words  must 
suffice.  The  variety  of  mechanism  is  one 
striking  feature.  Lungs  opening  by  a  glottis 
into  the  throat  are  the  most  usual  means  of 
obtaining  air.  The  Surface  skin  and  interior 
of  the  mouth,  however,  in  many  cases  act  as  a 
diffused  gill.  External  gills  are  present  on 
the  neck  of  a  few  primitive  newts  (Fig.  10). 


HOW  ANIMALS  BREATHE          117 

In  all  Amphibia  the  long  periods  of  immobility, 
and  the  deliberateness  of  movement  point  to 
a  lack  of  oxygen  and  to  imperfectly  developed 
muscles.  The  second  feature  in  amphibious 
breathing  is  the  absence  of  that  gulping  of 
water  so  characteristic  of  fish.  Only  in  the 
early  life  of  one  division  is  such  a  habit  prac- 
tised, and  here  it  is  associated  with  the  presence 
of  true  gills  and  gill-slits  on  the  sides  of  the  neck. 
By  such  means  the  tadpoles  of  most  frogs  and 
toads  obtain  their  supply  of  oxygen  ;  but  in 
no  grown-up  Amphibian  does  it  occur.  Further, 
no  swim-bladder,  as  such,  is  found  in  Amphibia, 
but  the  statement  only  shows  how  names  may 
blind  us  to  the  truth  of  facts.  If  we  call  an 
outgrowth  of  the  throat,  lungs  and  windpipe 
in  one  animal,  and  if  we  describe  it  as  air-bladder 
in  another,  we  may  miss  the  connection  between 
the  two.  The  fact  is  that  there  is  a  very  close 
resemblance  between  these  organs,  especially 
when  the  swim-bladder  of  the  Nile-fish,  Polyp- 
terus,  and  that  of  the  Mudfish  of  Queensland 
are  considered.  Physiologically  at  least  the 
lungs  of  Amphibia  and  the  swim-bladder  of 
such  fish  behave  very  much  alike.  But  there 
is  a  wide  difference  between  the  lung  of  a  frog 
filled  with  air  during  the  summer  and  unused 
during  the  winter,  and  the  swim-bladder  of 
an  active  fish  filled  frequently  with  almost 


118  THE  ANIMAL  WORLD 

pure  oxygen.  The  difference  in  behavior  is 
a  measure  of  the  diverse  quantities  of  energy- 
giving  oxygen  in  the  two  cases. 
f  Reptiles  have  not  improved  much  upon  the 
Amphibian  lungs.  They  have  lost  the  power 
of  breathing  through  the  skin,  and  all  traces 
of  gill-breathing  have  vanished  from  their  life- 
history.  Probably  the  extinct  flying  reptiles 
had  a  better  respiratory  system  than  the  modern 
ones.  Again,  we  see  that  slowness  of  lung- 
action  corresponds  to  slowness  of  movement 
or  to  spasmodic  quick  actions. 

RESPIRATION  IN  BIRDS.  —  Birds,  on  the  other 
hand,  are  the  most  strenuous  and  active  of  all 
things,  putting  not  only  energy  into  flight, 
hopping,  singing,  nesting  and  preening,  but 
into  clutches  of  eggs.  We  should  therefore 
expect  to  find  some  improvement  on  the  rep- 
tilian lungs,  and  we  do  find  it.  The  lungs  of 
birds  are  small  and  the  air-chambers  relatively 
limited  as  compared,  for  example,  with  mam- 
mals of  about  the  same  size  ;  but  air  is  drawn 
not  only  into  the  air-recesses  of  the  lungs,  but 
right  through  the  lungs  into  great  air-sacs  like 
those  of  flying  insects.  These  sacs,  unknown 
in  mammals  (if  we  except  the  vocal  sacs  of 
howling  monkeys  and  of  some  other  apes),  lie 
like  air-cushions  under  and  between  the  viscera, 
and  extend  in  many  birds  into  the  skin  and 


HOW  ANIMALS  BREATHE         119 

bones.  When  a  bird  expands  its  chest  and 
body,  air  rushes  through  the  lungs  into  these 
sacs  and  draws  with  it  a  great  draught  into 
the  breathing  spaces  (alveoli)  of  the  lungs  them- 
selves. The  air-sacs,  however,  have  no  blood- 
vessels, and  do  not  breathe  air.  When  the 
body  contracts  the  sacs  are  compressed  and 
their  contained  air  is  driven  out  as  a  rushing 
wind,  which  sweeps  out  through  the  lungs, 
carrying  with  it  the  carbon  dioxide  and  water- 
vapour  that  has  been  exhaled.  Thus  the  lungs 
of  a  bird  are  wholly  tidal  and  the  air-sacs,  while 
contributing  nothing  to  the  direct  aeration  of  the 
blood,  indirectly  determine  that  all  the  effective 
part  of  the  lungs  shall  be  completely  filled  and 
completely  emptied  each  successive  moment. 

MUSCLE  PIGMENTS.  —  Strange  to  say,  birds, 
unlike  fish,  have  no  large  oxygen  reservoir, 
at  least  none  has  yet  been  found:  but  nearly 
all  birds  have  in  their  dark-coloured  muscles 
a  pigment  which  acts  like  that  of  pelagic  fish 
as  a  store  of  oxygen,  and  enables  them  to  move 
constantly  and  to  continue  forcible  movements 
without  undue  fatigue.  The  unusual  fatigue 
and  powerlessness  of  migrant  birds  seen,  for 
example,  at  Heligoland,  at  sea  and  on  arrival 
from  a  distant  country,  is  probably  due  to  lack 
of  food  and  to  cold  rather  than  to  muscular 
fatigue  simply. 


120  THE  ANIMAL  WORLD 

LUNGS  OF  MAMMALS. — Mammals,  lastly,  have 
lungs  that  fit  them  for  the  deliberate  and  yet 
untiring  movements  so  characteristic  of  their 
order.  Their  lungs  are  not  swept  by  tides  of 
air.  The  upper  section  alone  receives  and  dis- 
charges air  with  each  rise  and  falling  of  the 
chest,  the  greater  part  of  their  capacity  being 
filled  with  stationary  air  which,  in  reality,  is 
diffusing  slowly  both  upwards  and  downwards; 
the  lungs  are  therefore  like  a  room  in  which 
air  is  constantly  changing  at  the  ventilators 
and  more  slowly  diffusing  at  the  centre. 

Fortunately,  however,  we  have  still  a  reserve 
supply  of  oxygen  upon  which  we  draw  with 
every  forced  movement.  Deliberate  people 
use  the  air  (or  rather  part  of  its  oxygen  content) 
that  diffuses  through  the  lungs,  as  we  all  do 
in  sleep  and  when  at  rest.  Directly  we  run, 
however,  and  possibly  even  under  the  mere 
influence  of  excitement  without  activity,  our 
lungs  take  on  a  new  mode  of  breathing  that 
adapts  them  to  the  enhanced  hunger  for  oxygen. 
As  Haldane  has  shown,  they  now  secrete  oxygen, 
and  breathing  proceeds  as  it  does  in  a  fish. 
Again,  the  profound  adaptiveness  and  reserve 
of  nature  is  strikingly  exhibited. 


THE    COLOURS    OF   ANIMALS      121 
CHAPTER    V 

THE    COLOURS    OF    ANIMALS 

THE  tints  and  colorations  of  animals  belong  to 
that  attractive  class  of  displays  which  interest  the 
most  as  well  as  the  least  scientific  observer. 

The  interest  of  colour,  like  that  of  form  and  of 
movement,  is  an  aesthetic  one,  and  a  peacock's 
tail  or  a  tiger's  skin  appeals  to  and  satisfies  our 
sense  of  beauty  without  raising  awkward  problems 
of  why  and  wherefore.  Much  in  nature  so  far 
outdistances  utility  as  to  discredit  the  principle 
of  explanation  in  the  minds  of  many  whose  voice 
would  be  heard  appealing  against  explanation  as 
diminishing  rather  than  increasing  their  enjoy- 
ment in  life.  The  fear  and  the  protest  are  really 
needless.  There  is  at  present  no  sign  that  we 
can  explain  anything.  What  is  gained  by  con- 
sidering colour  and  not  merely  gazing  at  it,  is 
appreciation  of  the  professional  standpoint;  we 
begin  to  see  how  to  observe.  Meaning  and  plan 
unnoticed  before  begin  to  interest  us.  We 
wonder  more  and  are  astonished  less. 

A  broad  survey  of  animal  as  of  vegetable  life 
discovers  the  prevalence  of  coloration.  Man 
himself  is  almost  the  only  pallid  thing,  and  bald- 
ness or  pallor  is  one  of  the  least  natural  products 


THE   ANIMAL   WORLD 

of  civilization.  The  wonder  of  savages  at  their 
first  introduction  to  white  men  is  really  the  most 
natural  and  justifiable  of  sensations;  and  the 
discarding  of  wigs  shows  a  decided  lack  in  one's 
sense  of  the  fitness  of  things.  In  this  matter,  as 
in  so  many  others,  women  have  the  right  intui- 
tion. If  animals  lack  colour  or  become  white 
(two  very  different  things)  they  generally  lie 
or  move  against  such  a  background  as  to 
be  inconspicuous.  The  transparent  Protozoan 
and  Medusa  lie  invisible  in  water,  the  white 
hare  and  arctic  fox  move  across  expanses  of 
snow. 

The  colouring  of  animals  is  due  to  two,  physi- 
cally, very  different  causes.  White  and  metallic 
colours,  tints  that  change  with  one's  point  of 
view,  mother-of-pearl  tints,  are  due  to  the  struc- 
ture of  the  surface  of  the  body,  that  is,  to  fine 
particles  or  to  thin  plates.  The  colour  may  be 
seen  best  against  a  dark  ground,  and  for  this 
purpose  underlying  pigment  may  be  present  but 
such  colours  do  not  arise  from  pigments.  On 
the  other  hand,  there  is  the  much  larger  class  of 
colour  effects  which  are  due  to  pigments.  It  is 
to  these  absorption-colours  that  the  following  re- 
marks apply. 

Two  CLASSES  OF  PIGMENTS. — Animal  pig- 
ments are,  speaking  generally,  of  two  classes 
differentiated  by  their  nature  and  especially  by 


THE  COLOURS  OF  ANIMALS      123 

their  varied  solubility.  The  most  abundant  class 
is  that  including  the  brown,  grey,  chocolate, 
yellow  and  red  tints  of  the  higher  animals.  The 
colours  of  hair  are  in  fact  as  well  as  in  name  a 
convenient  summary  of  such  pigments.  Wild 
animals  are  generally  only  hair-coloured,  that  is, 
their  skin  is  usually  pale  and  all  the  pigment  is 
present  in  the  hair,  feathers  and  eyes.  A  few, 
however,  have  both  skin  and  hair  coloured.  Loss 
of  hair  as  in  whales,  elephants,  rhinoceros  and 
man  is  accompanied  by  a  special  development  of 
brown  or  grey  pigment  in  the  skin. 

HAIR-COLOURING  OF  WILD  ANIMALS. — The 
most  general  fact  about  such  hair-colouring  is 
its  uniform  tone.  Leaving  aside  domesticated 
races  and  considering  wild  mammals  and  birds 
we  are  struck  by  the  general  brown  and  grey  tint 
or  dun  tint.  Rats,  mice,  rabbits,  foxes,  shrews, 
otters,  wild  horses,  wild  sheep,  wild  dogs,  wild 
cats,  monkeys,  bears,  lions,  most  deer  and  many 
others,  are  of  this  monotone  character.  Excep- 
tions there  are,  of  course,  in  the  form  of  tigers, 
zebras,  some  antelopes  and  civets,  but  these  fade 
into  insignificance  when  compared  with  the  vast 
numbers  of  the  monotonous  animals. 

Secondly,  the  monotone  is  really  misleading 
and  due  to  combinations  of  different  colours  in 
each  individual  hair.  A  grey  rat  or  rabbit,  for 
instance,  is  actually  parti-coloured,  but  the  pig- 


124  THE  ANIMAL  WORLD 

ments  are  so  closely  blended  as  to  produce  a 
general  grey  effect.  Examination  of  such  hair 
under  the  microscope  shows  that  the  grey  pig- 
ment is  not  uniformly  grey,  but  is  irregularly 
distributed  and  alternates  with  white  or  brown. 
The  coat,  in  fact,  is  of  many  colours. 

THE  SHADING  OF  ANIMALS. — Thirdly,  in 
such  wild  animals  a  scheme  of  colouring  exists 
and  is  such  that  the  upper  surfaces  of  the  body 
and  limbs  are  usually  darker  in  hue  than  their 
under  surfaces.  A  dark  back  and  a  pale  breast 
is  the  most  general  of  all  schemes.  This  contrast 
is  better  marked  in  short-legged  than  in  long- 
legged  animals,  for  a  reason  that  becomes  clear 
when  the  relations  of  light  and  shadow  to  animal 
coloration  are  considered.  Here  it  will  be  suffi- 
cient to  consider  shadow.  Shadow  throws  up 
an  object  and  renders  it  conspicouus.  If  by 
toning  the  back  dark  and  the  breast  light,  the 
shadow  of  the  upper  part  just  balances  the  dark 
tone,  the  effect  of  contrast  will  be  lessened  and 
the  animal  will  no  longer  be  so  apparent.  The 
principle  can  even  be  applied  to  individual  parts 
of  the  body;  a  projecting  brow  or  flank  can  be 
painted  out,  as  it  were,  by  lightening  the  hair 
just  beneath  it.  Such  marks,  therefore,  as  are 
seen  on  many  antelopes  are  not  real  contrast- 
colours,  but  are  efforts  to  efface  parts,  which  if 
coloured  uniformly,  would  stand  out  boldly. 


THE  COLOURS  OF  ANIMALS      125 

The  meaning  of  other  bars  and  stripes  is,  however, 
more  complicated. 

ALTERATION  OF  COLOURING  BY  DOMESTICA- 
TION.— The  effect  of  domestication  upon  animals 
is  to  alter  completely  these  rules  of  natural  color- 
ation. Domesticated  races  may  be  monotones, 
but  the  uniformity  is  real  and  not  a  coat  of  many 
colours  blending  into  one.  More  commonly 
they  are  spotted  or  blotched  in  a  bold  manner 
that  is  unknown  among  their  wild  ancestors. 
The  wild  cat  of  Egypt  is  yellow,  of  Great  Brit- 
ain grey  and  faintly  striped,  but  the  domestic 
cat  is  anything  from  black  to  white  through  a 
vast  gamut  of  colours  and  patterns.  The  wild 
dog  was  probably  grey  like  the  wolf  or  red  like 
the  Indian  Cyon,  but  the  modern  breeds  are 
either  true  monotones  or  spotted.  The  wild 
cattle  of  Europe  were  probably  brown,  the  wild 
horse  of  Asia  is  dun,  whereas  the  modern  cattle 
are  blotched  or  true  monotones,  and  horses  are 
frequently  spotted  with  a  curious  livery  colour. 
The  wild  boar  was  a  grey  of  the  true  speckly, 
mixed  tint.  The  modern  pig  is  pink,  black,  or 
parti-coloured.  The  goose  is  perhaps  the  only 
animal  that  has  not  changed  under  domestica- 
tion. Moreover,  the  "effacing  gradation"  of 
shading  from  dark  above  to  white  below  has  been 
abolished  by  human  selection;  and  we  have 
cattle,  dogs  and  fowls  that  can  be  seen 


126  THE  ANIMAL  WORLD 

from  far  in  consequence  of  this   lack  of  chest 
whiteness. 

COLORATION  OF  BIRDS  AND  REPTILES. — We 
must  now  consider  briefly  the  nature  and  arrange- 
ment of  pigments  in  birds  and  reptiles.  Their 
colouring  is  due  to  the  same  class  of  soluble 
substances,  the  so-called  "melanins"  we  have 
considered,  but  there  is  a  broad  distinction  be- 
tween the  colouring  of  birds  and  that  of  mammals. 
If  we  look  at  any  large  collection  we  notice  a 
broad  distinction  between  the  delicate  tracery 
of  the  plumage  in  birds  that  frequent  the  ground 
of  moor,  meadow  and  woodland,  and  the  contrast- 
ed plumage  of  the  more  active  perching  birds 
that  change  their  habitat  minute  by  minute.  As 
examples  of  these  compare  thrush,  snipe,  wood- 
cock, nightjar,  partridge  and  waders  generally, 
with  tits,  swallows,  finches.  Better  still  is  it  to 
compare  the  birds  in  the  open  and  to  note  the 
difficulty  of  seeing  "cock"  or  partridge  before 
flushing  them  as  compared  with  the  way  in  which 
ear  and  eye  are  drawn  to  finches  and  tits  by  their 
fussy  movements  and  conspicuous  colouring. 
The  speckled,  brownish  or  ashen  type  of  color- 
ation is  seen  also  in  reptiles,  particularly  in  snakes 
and  lizards.  In  fact,  speaking  generally  of  ani- 
mals, those  which  spend  long  intervals  motion- 
less possess  a  coloration  which  reproduces  in  a 
more  or  less  conventional  manner  the  play  of 


THE  COLOURS  OF  ANIMALS       127 

light  and  shadow  and  the  dominant  tone  of 
their  habitual  surroundings.  In  the  case  of 
sedentary  birds  as  the  American  artist,  Thayer, 
has  so  beautifully  shown  (p.  256),  the  "graining" 
reflects  pictures  of  shadow  under  foliage  with 
delicate  patterns  of  vegetation  drawn  across  it. 
The  peculiar  chain-like  patterns  of  snakes  are 
not  understood,  and  indeed  there  is  still  a  vast 
field  for  discovery  in  every  part  of  colour-phys- 
iology. Because  these  cases  of  effacing  colora- 
tion appeal  readily  to  our  sense  of  fitness,  they 
have  been  hailed  as  instances  of  "protective 
colouring,"  and  the  explanation  of  one  of  the 
most  complicated  of  colour-phenomena  is  held 
to  be  given  by  "protection"  which  in  animal 
as  in  human  economics  is  thought  to  meet  the 
needs  of  the  time.  But  when  the  magnificent 
range  of  such  harmonies  is  considered  both  in 
animal  and  plant  life,  of  which  colour-harmony 
is  only  one,  the  futility  of  such  a  ready-made 
solution  is  only  too  apparent.  There  is  a  fitness 
between  animal  life  and  its  surroundings.  The 
question  remains  how  close  is  the  fit,  and  that 
can  never  be  settled  if  we  remain  content  with  a 
vague  protective  phrase. 

FUNCTION  OF  DARK  PIGMENT. — Before  leaving 
the  Reptilia,  mention  should  be  made  of  the 
meaning  of  these  insoluble  pigments  to  which  all 
the  higher  animals  owe  their  colouring  from  the 


128  THE  ANIMAL  WORLD 

black  of  a  raven's  wing  to  the  grey  of  a  cat's 
whiskers.  We  have  seen  to  what  uses  they  may 
be  put  and  have  rapidly  glanced  at  some  of  the 
pictures  into  which  they  enter.  We  have  now 
to  ask  what  use  they  served  that  has  enabled 
such  pigments  to  be  so  lavishly  employed. 

One,  perhaps  the  chief  and  primary  use  of 
these  insoluble  and  variably  coloured  melanin- 
pigments  is  the  power  of  absorbing  the  ultra- 
violet rays  of  sunlight.  In  tropical  and  warm- 
temperate  countries,  bright  light  contains  a 
quantity  of  these  penetrating  and  chemically 
active  rays  as  well  as  heat-rays.  Fair  people 
rapidly  become  sunburnt  under  their  action,  and 
the  response  of  the  skin  in  becoming  brow^n  is  a 
defensive  one,  for  the  pigment  does  absorb  such 
rays  and  protects  the  deeper-lying  organs  from 
injury.  In  fact,  the  broad  classification  and  dis- 
tribution of  races  of  mankind  show  that  the  light 
and  dark  brown  races  inhabit  or  have  inhabited 
the  hotter  regions  of  the  world.  The  workers 
in  the  Indian  field  are  darker  than  those  of  the 
town.  The  natives  of  Australia  and  Africa  are 
darker  in  the  desert  regions  than  in  the  more 
temperate  zones.  On  the  other  hand,  light 
races  do  not  permanently  darken  when  living 
for  generations  in  the  tropics,  and  the  yellow 
Oriental  races  have  not  acquired  the  negroid 
tint  although  in  many  mongolian  countries  the 


THE  COLOURS  OF  ANIMALS       129 

'9 

amount  of  heat  is  equal  to  that  of  India  or  of 
Australia.  There  is,  therefore,  no  simple  rela- 
tion between  pigmentation  and  light-exposure, 
but  nevertheless  there  is  the  broad  association  of 
dark  native  colour,  with  a  dry,  hot  climate. 

Now  the  majority  of  temperate  animals  have 
as  we  saw  (p.  62-64),  come  from  warmer  or  sub- 
tropical countries  in  the  course  of  ages  or  are 
descended  from  an  epoch  in  which  what  are  now 
temperate  or  even  arctic  countries  were  then 
tropical.  Hence  the  persistence  of  dark  colouring 
in  many  animals  may  be  due  to  no  more  remote 
reason  than  conservatism,  the  tendency  of  the 
negro  to  remain  black  even  in  England  or  America, 
the  ultimate  explanation  of  blackness,  so  far  as 
we  can  now  see,  being  protection  from  ultra- 
violet light. 

ORIGIN  OF  PIGMENTS. — But  black  pigment 
(which  is  really  dark  brown)  is  due  to  a  substance 
which,  as  every  lady  knows,  can  with  a  little 
hydrogen  peroxide  be  transformed  into  brown, 
red  and  even  yellow.  These  shades  are  prob- 
ably phases  of  oxidation  of  a  "  mother  of  pig- 
ment" when  acted  upon  by  some  ferment.  The 
older  explanation  that  they  were  due  to  waste 
and  altered  blood  pigment  is  far  less  tenable. 
We  have  to  imagine  that  every  cat,  guinea-pig 
and  man  is  born  with  a  certain  amount  of  this 
colourless  "mother  of  pigment,"  this  "chrom- 


130  THE   ANIMAL  WORLD 

ogen"  as  it  is  called.  To  produce  colouring  a 
certain  ferment  must  act  upon  the  chromogen. 
In  the  best-known  case  this  ferment  is  a  wide- 
distributed  substance  known  as  "tyrosinase" 
(found,  for  instance,  in  the  skin  of  the  guinea-pig). 

Some  such  ferment-action  goes  on  in  many 
plants  and  animals,  and,  as  has  been  proved, 
often  takes  a  part  in  relation  to  oxygen  and  there- 
by initiating  oxydizing  changes.  When  the 
ferment  acts  upon  the  chromogen  it  produces  in 
the  latter  first  a  yellow,  then  an  orange,  after- 
wards a  reddish  and  finally  a  brown  tint,  the 
actual  colour  being  governed  probably  by  the 
amount  of  oxidation  to  which  the  chromogen  is 
subjected. 

COLOURING  OF  THE  RACES  OF  MEN. — We  can 
now  see  more  clearly  the  differences  and  the 
resemblances  between  the  races  of  mankind.  The 
white  races  have  chromogen  in  their  skin  and 
hair,  but  the  ferment  only  in  the  latter.  The 
yellow  races  have  some  of  each  in  the  skin,  much 
in  the  hair.  The  red  Indians  have  more  in  both 
skin  and  hair  and  the  Negroid  races  still  more  in 
both  situations.  In  domesticated  races  of  ani- 
mals the  distribution  of  these  factors,  which  to- 
gether give  rise  to  colour,  is  frequently,  as  we 
have  seen  (p.  125-6),  more  irregular  or  more 
uniform  than  in  wild  races.  A  fox-terrier,  Dutch 
rabbit,  Dalmatian  dog  and  tortoiseshell  cat 


THE  COLOURS  OF  ANIMALS         131 

show  that  the  primitive,  small-patterned  mar- 
bling has  given  place  to  a  large-patterned  chequer- 
ing, which  implies  that  the  chromogen  or  the 
ferment  is  absent  from  certain  areas.  The  zebras 
show,  however,  that  the  two  factors  may  occur 
in  definite  areas  in  a  wild  animal.  Further 
inquiry  is  necessary  to  determine  what  brings 
about  this  areolation  of  colour. 

WHITENING  OF  HAIR. — A  word  on  whiteness 
may  fitly  close  this  section.  We  know  that  white 
hair  develops  periodically  in  some  animals  such 
as  the  arctic-fox,  stoat  and  Alpine  hare;  that  in 
man  it  develops  only  after  maturity  and  to  vary- 
ing degrees  in  dark-haired  people :  and  that  whilst 
white  feathers  occur  sporadically  in  dark  garden 
birds  (blackbirds,  sparrows,  etc.),  true  albinos 
are  defective  animals,  often  deaf,  or  with  bad 
teeth  and  tender  eyes.  There  seem  to  be  good 
reasons  for  regarding  albinism  as  a  phenomenon 
perfectly  distinct  from  cases  of  partial  whiteness. 

This  white  colour  is  due  in  most  instances  to 
the  loss  of  pigment  in  the  hair  or  feathers,  and 
to  the  infiltration  by  air-bubbles  of  the  spaces 
previously  occupied  by  colouring  matter.  In 
few  grey  wild  animals  are  the  hair-shafts  free 
from  some  air-bubbles,  and  the  problem  of  arc- 
tic whitening  is  to  explain  the  seasonal  loss  of 
pigment.  The  most  feasible  solution  is  that  of 
Metschnikoff. 


132  THE  ANIMAL  WORLD 

According  to  the  distinguished  director  of  the 
Pasteur  Institute,  this  loss  of  pigment  is  occas- 
ioned by  the  immigration  of  colourless  blood- 
cells  from  the  hair-bulb  into  the  hair-shaft,  and 
by  the  ingestion  of  the  pigment  by  these  phag- 
ocytes. Following  upon  the  return  of  the  pig- 
ment to  the  hair-bulb,  bubbles  of  air  filter  into 
the  spaces  so  created  and  the  hair  becomes  grey 
or  white  according  to  the  activity  of  these  blood- 
cells.  It  is  probable,  however,  that  this  trans- 
location  of  pigment  is  only  one,  and  perhaps  not 
the  most  frequent,  mode  of  blanching. 

INFLUENCE  OF  LIGHT  ON  PIGMENT. — Loss  of 
pigment  either  periodically  or  sporadically  (as 
amongst  the  feathers  of  otherwise  dark  plumaged 
birds)  leads  naturally  to  the  question  of  the  re- 
lation of  light  and  of  darkness  to  the  develop- 
ment of  pigment.  There  is  a  vast  river  system, 
subterranean,  dark  and  cold,  in  the  earth's  crust. 
There  are  vast  abysses  in  the  ocean,  still,  cold, 
and  pressed  upon  by  the  weight  of  superincum- 
bent waters.  There  is  the  long  arctic  and  antarc- 
tic night  when  even  at  noon  one  cannot  see  to 
read:  and,  lastly,  there  are  habitats  chosen  by 
parasites,  whether  within  animal  or  plant  hosts, 
which  are  dark.  If  light  is  a  necessary  anteced- 
ent to  pigment  formation  we  should  expect  to 
find  cave,  abyssal,  and  parasitic  animals  unpig- 
mented;  and  we  should  not  expect  to  find  that 


THE  COLOURS  OF  ANIMALS        133 

pigment  can  develop  normally  in  the  interior  of 
animals.  There  is,  however,  the  possibility  that 
whilst  light  may  not  be  essential  to  the  formation 
of  pigment,  yet  that  the  tint  of  the  pigment  may 
be  governed  or  altered  by  the  action  of  light. 

COLOURING  OF  CAVE-ANIMALS. — The  most 
famous  subterranean  animal  is  the  Proteus  or 
blind  newt  of  the  Carinthian  Grotto.  This 
newt,  a  foot  in  length,  is  perfectly  white  except 
for  a  pair  of  red  gills  on  its  neck.  Its  eyes  have 
degenerated  and  are  covered  with  skin.  A  some- 
what similar  cave  newt,  Typhlomolge,  is  known 
from  caves  in  Texas,  and  this  again  is  colourless. 
There  is  also  an  interesting  fauna  of  cave  Crus- 
tacea, and  most  of  these,  again,  are  white  and 
blind.  The  nearest  allies  of  these  white  troglo- 
dytes are  the  coloured  newts  of  Europe  and  Amer- 
ica and  the  coloured  keen-eyed  Gammarus  of  our 
fresh- water  streams.  Hence  we  are  justified  in 
thinking  that  continued  darkness  has  led  to  the 
disappearance  of  this  pigment;  and  also  in  asso- 
ciating this  blanching  with  the  loss  of  eyesight. 
The  evidence  is,  however,  not  conclusive,  since  it 
is  possible  that  the  seeing,  coloured  newts  and 
shrimps,  might  be  those  which  found  their  way 
out  of  the  darkness,  and  that  the  blind  shrimps  and 
newts  are  the  original  stock.  The  experiment  of 
bringing  Proteus  into  the  light  shows,  however, 
that  the  opposite  series  of  events  has  probably 


134  THE    ANIMAL   WORLD 

occurred,  for  the  skin  of  this  animal  is  still  sensi- 
tive to  light  and  acquires  a  brown  colour  upon 
exposure;  it  behaves,  in  fact,  somewhat  as  a 
very  slow,  photographic  plate.  The  eyes,  how- 
ever, remain  disused  and  sight  is  not  reacquired. 
Hence  it  appears  likely  that  Proteus  and  its 
American  representative,  Typhlomolge,  are  de- 
scended from  seeing,  coloured  newts,  which  in 
the  old  and  new  world  respectively  have  been 
driven  by  stress  of  competition  into  darkness, 
blindness  and  pallor. 

COLOURING  OF  DEEP-SEA  ANIMALS. — Life  in 
darkness,  however,  does  not  always  involve  such 
degeneration.  The  abysses  of  the  sea  are  peopled 
by  a  variety  of  fish,  Crustacea,  molluscs  and 
other  animals.  The  colours  of  these  are  marked 
and  even  vivid.  The  deep-sea  fish  are  either 
black  or  brown,  the  Crustacea  are  scarlet,  the 
holothurians  (allies  of  the  shallow-water  form  of 
which  the  Chinese  make  their  edible  product,  tre- 
pang)  are  purple  or  brown,  the  corals  are  pink 
or  red.  There  are,  it  is  true,  a  certain  number 
of  blind  races  both  of  fish  and  of  Crustacea,  but, 
on  the  other  hand,  there  are  at  least  as  many  which 
have  enlarged  eyes,  and  blind  Crustacea  occur  in 
the  shallow  ocean  water  where  light  is  avail- 
able even  if  it  be  not  employed.  From  the  sur- 
face of  the  ocean  down  to  the  greatest  depths 
there  are  coloured,  seeing,  races  of  animals. 


THE  COLOURS  OF  ANIMALS      135 

In  explanation  of  this  singular  fact,  the  pres- 
ence of  phosphorescent  light  in  the  abysses  of 
the  ocean  has  been  adduced,  and  attempts  have 
been  made  to  show  that  the  depths  of  the  sea  are 
lit  up,  not,  it  is  true,  by  the  sun  (for  the  visible 
rays  of  the  sun  probably  do  not  extend  further 
than  200  fathoms  if  so  far),  but  by  light  given 
out  by  corals,  fish  and  Crustacea  whose  bodies 
are  girt  about  with  little  lamps  or  torches.  Just 
as  a  glow-worm  can  extinguish  its  light  so  it  is 
thought  that  these  deep-sea  animals  may  exer- 
cise some  control  over  their  lights.  In  some 
cases,  perhaps,  the  luminosity  is  given  out  con- 
tinuously. These  phosphorescent  organs  would 
at  least  light  up  the  surface  of  the  animal  that 
bears  them  and  might  therefore  compensate  for 
the  lack  of  sunlight. 

The  difficulty  of  testing  and  accepting  this 
attractive  suggestion  lies  in  our  ignorance  of 
what  happens  in  the  deep  sea.  We  know  that 
when  roughly  hauled  up  in  dredge  or  trawl  to 
the  surface,  these  abyssal  animals  glow  with 
fire  which  dies  after  a  while  and  can  be  rekin- 
dled by  agitation,  just  as  sparks  of  the  phospho- 
rescent light  are  emitted  from  many  shallow- 
water,  marine,  and  even  terrestrial  animals;  but 
we  do  not  know  how  constantly  or  how  intermit- 
tently this  light  is  given  out  when  the  lantern- 
bearers  are  living  under  normal  conditions.  It 


136  THE  ANIMAL  WORLD 

may  be  that  the  lantern-light  saves  their  eyes  and 
their  pigments  from  degeneration.  It  may  be 
that  ultra-violet  light  from  the  sun  penetrates 
far  deeper  into  the  sea  than  we  imagine.  It  may 
be  that,  as  the  deep  sea  is  peopled  by  the  migra- 
tion into  it  of  shallow-water  forms,  its  inhabi- 
tants have  retained  by  sheer  conservatism  the 
eyes  and  pigment  of  formerly  insolated  genera- 
tions, modified  but  not  extinguished  by  the  con- 
ditions of  the  abyss. 

Recent  experiments,  however,  tend  to  show  that 
absence  of  light  has  in  some  cases  a  rapid  blind- 
ing effect,  even  in  a  few  generations,  upon  animals 
accustomed  to  live  in  daylight.  After  a  few 
generations  kept  in  darkness,  the  eyes  of  Daphnia 
are  strikingly  modified  and  their  pigment  be- 
comes altered.  The  influence  of  prolonged  arctic 
darkness,  as  related  by  explorers,  shows  how 
great  a  part  light  plays  in  maintaining  health. 
All  internal  parasites  are  colourless,  whereas 
their  free-living,  sunlit  allies  are  pigmented.  On 
the  whole,  therefore,  we  are  bound  to  conclude, 
though  the  evidence  is  not  crucial,  that  absence 
of  light  favours  the  disappearance  of  pigment. 
The  development  of  eye-colour  in  darkness,  the 
formation  of  black  pigment  in  the  dark  interior 
of  many  animals,  warn  us,  however,  not  to 
accept  the  statement  that  the  formation  of  pig- 
ment is  dependent  upon  light.  In  our  present 


THE  COLOURS  OF  ANIMALS      137 

knowledge  we  must  say  that  pigment  can  arise 
independently  of  light,  that  the  melanic  pig- 
ments are  produced  by  the  interaction  of  a  fer- 
ment and  a  colourless  chromogen,  and  that  light 
may  determine  the  colour  or  tone  of  a  pigment. 
SOLUBLE  PIGMENTS. — The  dark  insoluble  pig- 
ments we  have  so  far  considered  are  especially 
characteristic  of  the  higher  Vertebrates,  though 
not  limited  to  these.  They  occur  also  in  Am- 
phibia and  fish  and  in  certain  Annelids;  but  in 
these  classes  the  melanic  pigments  are  associated 
with  lipochrome  or  fatty  pigments  of  an  entirely 
different  nature.  These  colouring  matters  are 
much  simpler  in  constitution;  in  their  purest  form 
being  a  hydrocarbon,  but  one  that  readily  oxi- 
dizes .  They  occur  in  four  tints,  red,  yellow,  purple, 
and  blue,  and  have  an  exceedingly  wide  dis- 
tribution both  in  animals  and  plants.  The  "yel~ 
low  spot  "  of  the  human  eye,  the  visual  purple  of 
the  retina,  the  yellow  yolk  of  eggs,  the  orange 
tint  of  a  carrot,  the  red  of  a  tomato,  are  all  due 
to  such  lipochromes.  These  pigments  are  as 
characteristically  soluble  in  alcohol  and  essential 
oils  as  the  melanins  are  insoluble.  Being  fre- 
quently dissolved  in  fatty  or  oily  media,  these 
pigments  are  also  found  in  many  reserve  prod- 
ucts, such  as  berries,  roots  or  tubers,  fungi  and 
eggs.  There  are  many  other  kinds  of  colouring 
matters  in  animals,  but  only  these  two,  the  mel- 


138  THE  ANIMAL  WORLD 

anins  and  the  lipochromes,  can  be  dealt  with  in 
this  work. 

COLOUR-CHANGES. — We  now  come  to  the 
peculiar  phenomena  known  as  change  of  colour 
that  are  exhibited  by  many  lizards,  amphibia, 
fish,  Crustacea,  cuttle-fish,  and  a  few  insects.  The 
most  striking  feature  of  this  manifestation  is  the 
property  of  altering  the  hue,  and  even  the  color- 
ation, of  the  body  in  a  short  period  of  time,  either 
in  a  few  seconds  or  at  definite  periodically-re- 
curring intervals  of  a  few  hours.  This  change  of 
colour  differs  markedly  from  the  seasonal  changes 
of  plumage  or  pelage,  and  also  from  the  gradual 
colour-change  in  many  insect-larvae  when  placed 
on  contrasted  surroundings.  A  small  cuttlefish 
such  as  can  be  taken  with  a  shrimp  net  at  low 
tide  on  our  southern  sandy  bays,  exhibits  the 
momentary  changes  in  a  most  striking  manner. 
At  the  least  agitation  its  body  blushes  a  reddish 
brown,  and  then  instantly  fades  to  an  intense 
pallor,  followed  by  another  blush.  Shrimps  and 
prawns  transferred  from  a  dark-bottomed  dish 
of  sea-water  to  a  white  vessel  speedily  alter  from 
a  sandy  or  red-lined  pattern  to  a  transparent  and 
almost  colourless  state.  Plaice  and  many  other 
bottom  fish  have  the  same  property  of  rapidly 
altering  their  tone.  Heat  and  cold,  moisture  and 
dry  ness,  softness  and  roughness  call  forth  similar 
rapid  changes  of  colour  in  frogs  and  some  lizards. 


THE  COLOUR'S  OF  ANIMALS     139 

RHYTHMICAL  COLOUR-CHANGES. — More  signifi- 
cant than  these  temporary  changes  of  colour  are 
those  rhythmically  recurring  phases  that  accom- 
pany sleep  and  waking.  Many  animals  sleep  by 
day,  others  by  night.  Like  flowers,  they  do  not 
need  the  awakening  touch  of  nightfall  and  day- 
break in  order  to  quicken  their  pulse  and  set 
them  going.  Light  and  darkness  have  played 
alternately  so  long  upon  their  nervous  system  as 
to  produce  a  rhythmic  habit  of  action  and  repose 
which  is  helped,  but  not  begun,  by  daily  and 
nightly  recurrence.  So  the  ^Esop-prawn  Hippo- 
lyte  sleeps  on  the  sea- weed  of  its  choice  during  the 
day,  even  if  we  plunge  it  in  a  dark  chamber,  nor 
does  it  fail  to  awake  at  evening  though  we  turn 
its  night  into  day.  Now  this  "periodicity  "  is 
expressed  in  the  colour  of  the  body.  The  wakeful 
hours  of  Hippolyte  are  hours  of  expansion.  The 
red  and  yellow  pigments  flow  out  in  myriads  of 
stars  or  pigment-cells;  and  according  to  the 
nature  of  the  background,  so  is  the  mixture  of 
the  pigments  compounded  to  form  a  close  repro- 
duction both  of  its  colour  and  its  pattern;  brown 
on  brown  weed,  green  on  ulva  or  eel-grass,  red  on 
the  red  algae,  speckled  on  the  filmy  ones.  A  sweep 
of  a  shrimp  net  detaches  a  battalion  of  these 
sleeping  prawns,  and  if  we  turn  the  motley  into  a 
dish  and  give  a  choice  of  seaweed,  each  variety 
after  its  kind  will  select  the  one  with  which  it 


140  THE  ANIMAL  WORLD 

agrees  in  colour  and  vanish.  At  nightfall  Hippo- 
lyte,  of  whatever  colour,  changes  to  a  transparent 
azure  blue;  its  stolidity  gives  place  to  a  nervous 
restlessness;  at  the  least  tremor  it  leaps  violently 
and  often  swims  actively  from  one  food-plant  to 
another.  This  blue  fit  lasts  till  daybreak,  and  is 
then  succeeded  by  the  prawn's  diurnal  tint.  Thus 
the  colour  of  an  animal  may  express  a  nervous 
rhythm. 

Such  reflection  of  the  inner  states  by  outward 
show,  is  seen  in  many  lizards,  fish,  cuttlefish,  and 
even  in  some  insects.  In  these  sensitive  animals, 
the  sleeping  state  is  usually  expressed  by  pallor, 
the  wakeful  condition  by  dark  colouring. 

Lastly,  the  question  of  the  importance  of  pig- 
ments to  animal  life  calls  for  notice.  We  know 
that  such  pigments  as  we  have  here  dealt  with, 
absorb  light,  but  there  are  no  known  processes  in 
animal  economy  for  which  light  is  an  essential  fac- 
tor except  sight,  and  we  cannot  suppose  every 
pigment  spot  to  be  a  means  of  distinguishing 
darkness  from  light.  The  analogy  of  plants  sug- 
gests that  pigments  play  many  parts,  that  they 
may  render  easier  some  task  that  is  only  rarely 
performed  without  them,  and  that  they  are  fac- 
tors in  that  refined  and  intimate  associateship 
between  living  energetics  and  inanimate  forces  of 
which  biology  is  only  beginning  to  form  a  con- 
ception. 


THE    SENSES    OF   ANIMALS       141 
CHAPTER  VI 

THE   SENSES   OF   ANIMALS 

ANIMATE  AND  INANIMATE  BEHAVIOUR. — One 
great  difference  between  beings  and  inanimate 
substances  lies  in  their  behaviour.  We  speak 
of  the  reciprocal  behaviour  of  certain  substances 
as  a  chemical  reaction,  and  are  beginning  to  re- 
alize how  profoundly  conditions  may  modify  that 
behaviour;  how  what  is  true  of  an  experiment 
carried  out  with  ordinary  moist  air,  may  never 
occur  in  dry  air,  or  vice  versa;  how  impurities  such 
as  lithium  are  in  every  bit  of  glass  apparatus  and 
may  be  a  condition  of  a  certain  result  which 
would  never  arrive  if  perfectly  pure  silica  were 
used;  how  minute  traces  of  a  substance  may 
have  a  determining  effect  on  the  final  result.  In 
an  organism  there  are  means  of  detecting,  and 
often  of  expressing  an  attitude  towards,  these 
conditions  of  reaction.  Above  all,  there  is  in 
a  being  not  only  a  certain  awareness,  but  a  cer- 
tain power  of  choice,  a  certain  independence 
when  faced  by  a  multitude  of  alternatives.  The 
movement  of  a  barometer  and  of  a  swallow  rise 
and  fall  together,  but  the  glass  is  simply  inertly 
submitting  to  atmospheric  pressure,  whilst  the 
bird  is  aware  of  a  thousand  changes:  the  in- 


142  THE  ANIMAL  WORLD 

creased  moisture  and  other  conditions  of  the  air, 
the  altered  abundance  and  distribution  of  in- 
sects, and  no  doubt  many  other  changes  of  which 
we  have  no  conception.  To  put  the  matter  very 
briefly,  the  animal  is  pressed  upon  (though  often 
not  to  the  point  of  consciousness)  by  altering 
waves  of  light,  heat,  degrees  of  moisture,  of 
gravity,  hardness,  odours,  chemical  substances. 
Some  of  these  waves  make  no  more  impression 
upon  it  than  does  the  sea  on  a  rock,  or  the  wrong 
key  on  a  lock;  others  find  a  responsive  reception. 
A  key  has  been  found  that  fits  some  lock  and 
the  bolt  shoots  back.  In  other  words,  move- 
ment is  a  response  to  a  particular  stimulus.  Not 
only  movement;  but  internal,  as  well  as  external, 
change  of  any  kind  is  set  going  by  the  existence 
of  this  exclusive  sympathy  between  the  proper- 
ties of  beings  and  the  keys  that  unlock  them.  In 
some  animals,  as  in  clocks,  only  one  key  sets  the 
movements  going;  in  others,  several.  In  our- 
selves memory  and  association  are  more  powerful 
than  actual  change  of  current  order  in  producing 
an  effect. 

UNDER  LOCK  AND  KEY. — The  sense  organs 
and  the  nervous  system  form  together  the  locks 
which  determine  animal  behaviour.  If  we  try 
the  keys  of  light,  moisture,  odour,  or  food  upon 
the  ear  the  bolt  remains  fixed;  the  right  key  is 
the  key  of  sound,  and  this  is  again  a  whole  gamut: 


THE  SENSES  OF  ANIMALS        143 

some  notes  too  low  for  us  to  hear,  some  too  high. 
To  some  we  remain  inert,  while  others  raise  pas- 
sions and  win  battles.  The  lower  consciousness, 
roused  from  its  sleep,  becomes  in  turn  a  key  that 
unlocks  our  energy.  So  with  animals,  the  varying 
play  of  the  complicated  inorganic  world  calls 
forth  in  them  no  chaotic  and  inconstant  changes, 
but  an  orderly  sequence  of  responses  as  definite 
as  the  classification  into  which  their  structure 
falls.  Behaviour  is  organized  and  individualized. 

This  harmonious  result  is  far  beyond  a  complete 
analysis.  It  is  often  attained  by  the  nervous  sys- 
tem acting  upon  the  body  under  the  stimuli  of 
the  sense  organs.  But  it  is  realized  in  Proto- 
zoa, in  animals  that  have  no  organs  of  any  kind. 
Adaptive  response  is,  in  fact,  a  quality  of  living 
matter,  just  as  much  a  property  as  is  its  compli- 
cated chemical  composition  or  its  structural  cells 
and  nuclei. 

THE  OUTER  WORLD  AND  THE  SENSORY  WORLD. 
— The  outer  world,  in  a  magnificent  fulness  and 
variety,  beats  upon  all  living  things,  but  sensa- 
tion of  that  fulness  and  variety  is  felt  by  few. 
Where  there  is  no  central  nervous  system,  reac- 
tion can  hardly  be  conscious.  Some  Protozoa, 
for  example,  respond  to  different  rays  of  light,  such 
as  red  and  green,  by  a  differential  movement,  but 
they  do  not  perceive  the  light  as  red  or  green. 
Many  seek  their  food  by  responding  to  the 


144  THE  ANIMAL  WORLD 

presence  of  carbonic  acid  in  the  water,  for  by 
moving  towards  that  acid  they  are  brought  into 
contact  with  bacteria  upon  which  they  live, 
but  they  do  not  perceive  the  gas.  The  Amoeba 
selects  from  a  mass  of  algse  one  or  two  forms  upon 
which  it  feeds  by  flowing  round  them  and  enclos- 
ing them  by  its  mobile  body — performing,  in 
fact,  the  exact  opposite  of  the  process  in  ourselves, 
for  whilst  the  food  passes  into  us,  Amoeba  gets 
outside  its  food.  But  this  constant  handling  of 
oval  or  rectangular  algse  gives  no  sense  of  form  to 
Amoeba.  These  green  cells  make  no  colour-sen- 
sation upon  it.  The  cold  or  heat  of  the  water  de- 
termines its  activity  or  stillness,  but  produces 
no  sense  of  what  we  call  heat  or  cold.  These 
must  be  in  all  animals  some  inner  world  unlocked 
by  the  keys  to  which  they  respond,  but  it  is  a 
world  utterly  different  from  that  active  world  of 
fulness  and  variety  as  we  know  it,  and  as  differ- 
ent from  the  perceptions  which  it  awakens  in  us. 
The  unconscious  world  or  state  which  is  unlocked 
by  touch,  gas,  light,  and  so  on,  is  only  known  to 
us  by  the  responses  which  animals  make.  This 
constitutes  their  world.  Towards  all  other  stim- 
uli or  beating  of  the  environment  they  are  utterly 
unresponsive.  These,  however  few  they  may  be, 
suffice  for  their  needs.  We  may  call  this  poverty- 
stricken  collection  of  sensations  the  Sensory 
world,  so  long  as  we  understand  by  that  term,  not 


THE  SENSES  OF  ANIMALS        145 

perception,  but  such  a  dull  and  unilluminating 
sense  as  our  own  sense  of  temperature,  hunger, 
or  pain. 

RESPONSES  OF  SIMPLE  ANIMALS. — This  sensory 
world,  though  absolutely  different  from  the 
outer  one,  is  so  delicately  adjusted  to  the  mo- 
ment and  behaviour  of  animals  as  to  guide  them 
safely  in  an  environment  of  which  they  know 
nothing.  The  Paramecium,  or  "slipper-animal- 
cule," has  three  adjustments  or  threads  by 
which,  as  it  were,  it  hangs  in  a  treacherous  and 
threatening  world  of  which  it  realizes  nothing. 

MEDUSA. — The  Rhizostoma  (a  medusa)  has 
only  one  response,  namely,  to  wave-vibration, 
yet  so  adequately  adjusted  are  its  movements 
to  its  needs  that  the  muscular  response  to  waves 
maintains  it  in  a  quiet  stratum  of  water,  draws 
food  into  its  mouth,  and  aerates  its  tissues. 
This  response  is  effected  by  the  little  weighted 
sense-organs  that  occur  at  intervals  round  the 
bell-margin  and  communicate  with  the  muscles 
by  the  aid,  and  under  the  governance,  of  the 
nerve-fibres  that  encircle  the  margin  of  the  bell. 
This  stimulus  transmitted  by  the  sense-clubs 
from  the  agitated  water  is  apparently  the  only 
key  that  unlocks  this  jelly-fish.  The  sensory 
world  is  reduced  almost  to  the  point  of 
Nirvana. 

SENSES  OF  EARTH-WORMS. — The  earth-worm 


146  THE  ANIMAL  WORLD 

lives  a  much  fuller  life.  It  is  the  first  animal  to 
acquire  a  sense  of  form,  as  Darwin  showed  by 
noticing  the  way  in  which  leaves  and  paper  cut 
in  various  shapes  were  handled  by  the  lips  of 
worms  and  then  drawn  into  their  burrows. 
Though  having  no  eyes,  these  animals  distinguish 
readily  between  light  and  darkness,  reaching  out 
of  their  burrows  by  night  and  withdrawing  into 
them  again  when  day  dawns.  Heat  and  cold  are 
stimuli  to  which  appropriate  responses  are  made: 
warmth  causing  earth-worms  to  ascend,  cold 
causing  them  to  descend  and  so  to  escape  from 
frost.  Intense  heat  is  avoided,  like  intense  cold, 
by  prolonging  the  burrow  to  a  great  depth.  Some 
earth-worms  also  show  a  capacity  for  making 
unusually  violent  bounding  movements  in  the 
effort  to  escape  from  moles. 

NERVOUS  SYSTEM  OF  WORMS. — The  nervous 
system  which  facilitates  and  governs  these 
various  responses  consists  of  a  double  chain  of 
nerve-knots  or  ganglia  placed  on  the  ventral 
surface,  with  the  exception  of  the  first  pair  or 
"brain"  which  lies  above  the  mouth  and  inner- 
vates the  lips.  No  sense-organs  more  complex 
than  scattered  touch-cells  are  present,  showing 
that  an  animal  may  live  a  fairly  full  life  without 
possessing  any  of  the  highly  organized  sense- 
organs  that  keep  ourselves  informed  of  change 
in  the  outer  world. 


THE  SENSES  OF  ANIMALS        147 

SENSES  OF  ARTHROPODS. — The  Arthropods 
and  the  Molluscs  are  the  typical  examples  of 
Invertebrates  in  which  a  "brain"  is  developed. 
By  a  "brain"  is  meant  that  part  of  the  nervous 
system  in  which  the  sensory  world,  unlocked  by 
the  keys  of  appropriate  nervous  stimuli,  has  its 
especial  seat.  It  is  the  nerve  centre  in  which 
sensations  may  rise  to  the  point  of  perceptions, 
and  in  which,  therefore,  consciousness  is  born. 
In  addition  to  these  properties,  the  brain  pos- 
sesses in  a  high  degree  the  oldest  property  of 
nervous  tissue,  namely,  the  governance  of  the 
body  along  the  lines  of  adaptive  muscular  re- 
sponse. 

Arthropods  possess  the  sense  of  sight,  and  even 
of  colour-vision — or  at  least  the  property  of 
choosing  certain  rays  of  light  and  of  avoiding 
others.  Thus  if  a  spectrum  be  placed  over  a 
long  water-trough  containing  a  number  of  Daph- 
nia, the  animals  will  congregate  under  the  green 
rays.  They  will  move  towards  a  source  of  light 
somewhat  as  a  moth  makes  furiously  for  a  candle. 
This  response,  like  all  the  rest,  is  an  adaptive 
one.  Daphnia  feeds  on  minute  floating  algae 
which  can  only  grow  and  multiply  in  light.  Hence 
by  moving  towards  the  source  of  light  Daphnia 
is  unlike  the  moth,  making  for  the  likeliest  source 
of  food. 

This  movement  is,  however,  carried  out  by 


148  THE  ANIMAL  WORLD 

Paramecia  and  most  plants,  and  is  therefore  inde- 
pendent of  eyes  or  of  any  sense-organs;  but  it 
does  not  follow  that  the  presence  of  some  form  of 
eye  may  not  be  an  advantage.  The  quicker  the 
response,  the  more  rapidly  will  food  be  obtained; 
the  more  quickly  also  will  the  next  generation 
be  developed.  Daphnia,  however,  like  all  Arthro- 
pods, has  two  kinds  of  eyes :  a  simple  eye  in  the 
middle  of  its  "forehead,"  and  a  pair  of  compound 
eyes  so  nearly  fixed  together  in  the  middle  line 
as  to  appear  like  one;  the  latter  are  constantly 
vibrating.  The  meaning  of  these  "ocelli,"  or 
simple  eyes,  in  addition  to  the  compound  eyes 
has  long  been  a  puzzle  to  naturalists.  These 
two  contrasted  types  of  eye-like  structure  occur 
in  all  Crustacea,  insects,  Arachnids  (that  is, 
spiders,  scorpions,  king-crabs,  and  mites)  and 
millipedes.  Nor  does  the  puzzle  end  there,  for 
all  vertebrates,  even  man  himself,  have,  in  ad- 
dition to  the  paired  functional  eyes,  a  cone- 
shaped  body — the  pineal  body — which  acquires 
in  some  fish  and  lizards  an  eye-like  structure. 
This  so-called  pineal  eye  is  closely  attached  at 
one  end  to  the  brain  and  underlies  a  little  clear 
scale  on  the  roof  of  the  head.  The  function  of 
this  third  "eye"  is  at  present  unknown. 

SIGHT  IN  ARTHROPODS. — Coming  back  to 
Daphnia  and  its  allies,  the  development  of  the 
paired  compound  eyes  in  Arthropods,  espe- 


THE  SENSES  OF  ANIMALS        149 

cially  in  insects,  is  carried  to  a  high  degree  of 
complexity.  Photographs  taken  through  the 
eye  of  a  dragon-fly  show  that,  though  the  eye 
is  compounded  of  many  lenses  and  sensitive 
areas  (retinulae)  corresponding  to  them,  yet 
the  whole  eye  throws  one  image  on  to  the  retina. 
However  complex  such  an  eye  may  be,  it  is 
devoid  of  any  focussing  arrangement  and  can 
only  receive  a  clear  image  when  the  retina  and 
the  object  are  separated  by  the  focal  length  of 
the  lenses.  Hence  the  need  for  active  movement 
on  the  part  of  such  eye-bearers.  Either  they 
must  move  their  eyes  to  and  fro  or  (by  far  the 
more  common  plan)  search  actively.  The  in- 
cessant and  fussy  activity  of  insects  thus  receives 
some  explanation.  But  when  the  image,  say 
of  some  flower  is  clearly  defined  upon  a  bee's 
eye  we  have  no  right  to  think  that  it  means  to 
the  bee  what  it  does  to  us,  for  we  know  that  we 
interpret  sight  by  touch.  An  insect's  clear  sight 
is  limited  by  a  few  inches  or  feet,  and  its  inter- 
pretation of  the  images  gained  at  that  effective 
distance  is  probably  in  proportion  to  its  tactile 
sensations.  These  touch-sensations  are  prob- 
ably very  acute  and  take  a  far  larger  share  in 
the  inner,  or  sensory,  world  of  Arthropods  than 
in  our  own,  though  whether  they  rise  to  the 
threshold  of  consciousness  as  perceptions  seems 
at  least  doubtful.  Colour  appreciation  is  a  vexed 


150  THE  ANIMAL  WORLD 

question,  but  there  certainly  seems  a  relation 
between  the  colours  of  flowers  and  the  attraction 
that  they  have  for  bees.  Movement  of  an  object 
across  the  field  of  sight  instantly  arouses  insect 
activity,  and  even  a  shadow  falling  upon  a  rest- 
ing butterfly  startles  it  at  once. 

SMELL  IN  ARTHROPODS  . — The  sense  of  smell 
is  also  very  acute  in  insects,  and  probably  in 
Arthropods  generally.  Thus  ants  of  a  given 
colony  have  each  their  nest-smell  by  which  they 
recognize  one  another  and  distinguish  and  harass 
exactly  similar  fellows  from  a  neighbouring  nest 
which  are  tarred  by  a  different  brush.  Moths  and 
butterflies  mate  by  means  of  this  sense.  The 
male  in  many  species  is  provided  with  beautifully 
plumose  antennse  covered  with  hundreds  of 
olfactory  hairs.  These  hairs  are  in  close  associ- 
ation with  the  "brain"  by  a  system  of  nerves,  and 
the  brain  in  turn  controls  the  muscles  of  flight. 
The  female  of  such  insects  has  no  such  special 
endowments,  and  may  even  be  deprived  of  wings 
and  wing  muscles,  but  she  has  a  special  odour- 
producing  gland  near  the  tip  of  the  tail,  and  the 
faint  scent  from  this  gland  (to  which  it  need 
hardly  be  said  we  are  utterly  impervious,  so 
unresponsive  is  our  nose  to  myriads  of  impres- 
sions that  lower  animals  receive  and  respond 
to  with  ease)  is  carried  by  damp  gentle  breezes 
over  the  countryside  in  an  extremely  attenuated 


THE  SENSES  OF  ANIMALS        151 

form.  Yet  in  this  almost  incredibly  diluted  state 
the  scent  of  the  oak-eggar  moth  or  of  the  va- 
pourer  is  not  imperceptible,  and  if  there  is  a  male 
in  the  district  he  will  soon  be  found  by  exposing 
a  female  on  a  mild  damp  evening.  This  art  is 
called  "sembling"  (assembling)  and  is  a  com- 
monplace of  entomological  procedure. 

SENSE  OF  LOCALITY. — Such  fineness  of  sensa- 
tion suggests  that  many  insects  carry  out  their 
responses  to  a  degree  of  delicacy  unknown  to 
us.  A  bee  or  a  wasp  circles  round  the  neigh- 
bourhood of  her  nest  and  seems  to  fix  the  situation 
of  each  object  in  her  "memory"  so  that  she  can 
find  her  way  back  again  with  ease.  Even  if  the 
nest  be  moved  from  the  starting-point  of  her 
morning  flight  she  will  return  to  the  exact  spot 
and  appear  surprised  to  find  it  gone.  Probably 
our  world  of  perception,  and  still  more  that  other 
world  of  conception,  is  too  much  with  us  to  allow 
us  to  print  sensation  upon  our  memory  as  does 
a  bee  or  a  wasp. 

THE  PERCEPTUAL  WORLD. — At  what  point 
in  animal  organization  does  this  world  become 
real?  What  degree  of  refinement  of  sensation,  or 
what  multiplicity  of  sensation,  creates  the  faculty 
for  perceiving  the  outer  world?  What  still 
greater  civilization  is  necessary  before  con- 
structions can  take  place  in  the  perceptual 
mind  and  a  new,  wild  world  of  conception  arise 


152  THE  ANIMAL  WORLD 

within?  A  few  suggestions  are  all  that  can  be 
given  in  this  work  in  reference  to  such  far-reach- 
ing questions. 

CONDITIONS  FOB  PERCEPTION. — With  regard 
to  perception:  high  quality  as  well  as  the  quan- 
tity of  the  nervous  system,  and,  probably,  es- 
pecially of  that  part  which  we  call  brain,  is  an 
essential  preliminary.  One  important  factor 
in  determining  this  result  is  the  reaction  of  the 
body  itself  upon  the  nervous  system.  We  are 
too  apt  to  think  of  sensation  as  dependent  only 
upon  a  certain  stimulus  set  up  in  nerves  by 
change  of  environment,  and  we  forget  too  readily 
that  the  effect  of  this  stimulus  depends,  not  only 
upon  the  external  world,  but  upon  the  receptive- 
ness  of  the  nerves.  In  sleep,  noises,  odours,  even 
"pains"  fail  or  may  fail  to  wake  us — that  is, 
they  fall  upon  our  nerves  in  an  unreceptive  mood. 
Thinking  over  this  "moodiness"  of  living  beings, 
we  suddenly  see  that  from  plants  right  up  to 
the  highest  animals,  periods  of  nervous  receptive- 
ness  and  of  dulness  recur.  Night  and  day,  as 
we  have  seen,  are  often  accompanied  by  alterna- 
tive phases  of  activity  and  repose.  Winter  and 
summer  on  land  and  in  water  are  again  periods 
of  rest  and  responsiveness.  These  rhythmic 
alternations  set  up  a  rhythm  in  the  nervous 
system  to  some  extent  independent  of  the  external 
world.  Not  only  is  a  sort  of  memory  thus  es- 


THE  SENSES  OF  ANIMALS        153 

tablished,  but  the  body  itself  reacts  upon  the 
nervous  system  at  certain  periods.  Many  chil- 
dren only  develop  late;  they  need  some  stimulus 
which  has  to  proceed  from  their  glands  and 
muscles  and  has  to  act  upon  the  nervous  system 
before  this  becomes  fully  receptive.  Some 
children  are  temporarily,  some  remain  perma- 
nently "feeble-minded";  there  is  an  arrest  of 
development  in  such  cases;  perception  there 
may  be,  conception  is  limited  and  difficult.  In 
curable  cases  an  extract  of  a  gland  in  which  these 
subjects  are  deficient  in  quality,  and  often  in 
quantity,  may  do  good.  The  blood  and  the 
nerves  respectively  carry  from  the  body  to  the 
central  nervous  system,  material  and  impulse 
which  assist  growth,  create  "tone,"  and  pave 
the  way  for  further  fineness  of  sensation  and  of 
perception. 

It  is  probably  to  these  unconscious  impulses, 
rhythmic  or  unperiodic,  which  flow  from  the 
body  to  the  nervous  system,  that  the  perceptive- 
ness  of  the  brain  owes  most.  "Sensations"  of 
sound,  sight,  smell,  and  so  on,  may  occur  without 
consciousness  being  aroused;  we  are  still  in  the 
sensory  world.  This  is  probably  the  case  with 
all  invertebrates  save  the  higher  arthropods 
and  cuttle-fish.  But  even  these  imperceptive 
animals  are  subject  both  to  the  action  of  the 
outer  world  and  to  the  reaction  of  their  own 


154  THE  ANIMAL  WORLD 

muscles  and  organs  upon  their  nervous  system. 
These  stimuli  from  without  and  from  within 
play  constantly  or  rhythmically  upon  the  ner- 
vous system  and  form  the  basis  upon  which 
perception  is  founded.  Perception  is  a  new  world, 
but  it  is  unlocked  by  the  same  keys  as  those 
which  admit  to  the  sensory  world. 

THE  CONCEPTUAL  WORLD. — Little  as  we  know 
of  the  conditions  of  perceiving  the  outer  world, 
we  know  still  less  of  the  means  by  which  we  build 
up  our  perceptions  with  a  new  constructive 
synthesis  and  call  them  conceptions.  Yet  here 
is  the  world  in  which  we  are  at  home.  Our  senses 
are  less  acute  than  those  of  most  animals  and 
we  are  often  deprived  of  one  or  more  of  them 
from  or  soon  after  birth.  Our  perceptions  may 
therefore  be  far  less  extensive  than  those  of  the 
higher  animals  in  which  they  are  stored  by  the 
quest  for  food,  the  avoidance  of  enemies,  and 
the  manifold  rain  of  impulses.  Yet  under  all 
these  disadvantages  we  learn  and  make  our 
conceptions  quickly,  and  are  ruled  by  them  all 
our  lives.  Unconscious  of  perceptions,  often 
blinded  by  our  "notions"  to  the  form  and  even 
the  existence  of  things  about  us,  we  make  our 
own  world — often,  indeed,  are  not  happy  until 
we  live  in  an  artificial  world  of  our  own  con- 
struction. Perhaps  a  dog  chewing  the  cud  of 
the  day's  reflections  is  lifted  up  into  a  conceptual 


SOCIETIES    AND    ASSOCIATIONS    155 

world    founded    on    olfactory    experiences    and 
dreams  of  smells. 


CHAPTER  VII 

SOCIETIES  AND  ASSOCIATIONS  I     SYMBIOSIS 

THE  study  of  animal  life  is  apt  to  take  the 
standpoint  of  the  individual  and  to  consider 
isolated  structure  and  isolated  behaviour.  Even 
the  poets  tell  us  that  we  live  alone  and  enisled. 
Beings,  however,  only  exist  in  relation,  and 
their  relationships  take  the  form,  not  only  of 
dependence  upon  the  inanimate  world,  but  of 
a  social  order.  Some  animals,  such  as  the  ant, 
cannot  exist  alone;  no  solitary  ants  are  known. 
In  other  orders,  such  as  bees  and  wasps,  the 
more  primitive  members  are  solitary,  the  more 
advanced  social.  Many  birds  aggregate  in  huge 
flocks  during  the  winter  and  break  up  at  the 
breeding  season  into  pairs.  Many  caterpillars 
are  social,  and  spin  a  common  web  for  protection. 

SYMBIOSIS. — But  not  only  do  we  find  societies, 
and  even  castes,  amongst  animals;  close  study  of 
animal  structure  serves  to  show  that  even  the 
individual  is  frequently  an  association  of  two 
independent  organisms  living  together  in  some 
kind  of  partnership.  The  classical  example  of 
this  partnership,  or  "symbiosis,"  is  that  of 


156  THE  ANIMAL  WORLD 

lichens.  These  plants,  ever  since  De  Bary's 
discovery,  have  been  recognized  as  an  association 
of  two  entirely  independent  and  dissimilar  organ- 
isms— algse  and  fungi.  We  now  know  that  many 
trees  and  flowers  only  flourish  if  they  have  a  cer- 
tain fungus  attached  to  their  roots,  and  hence 
the  need  of  removing  some  of  the  earth  with  the 
roots  of  certain  plants  when  transplanting  them 
from  one  site  to  another.  Alders,  orchids,  and 
lilies  are  examples  of  this  kind.  Apart  from 
cases  of  sheer  parasitism,  the  most  prevalent 
association  between  one  form  of  being  and 
another  is  probably  the  occurrence  of  bacteria. 
Bacteria  are  of  many  kinds:  some  deadly, 
some  innocuous,  some  actually  helpful.  The 
helpful  kinds  are  chiefly  those  which  have  a 
digestive  action  upon  tough  substances  taken 
into  the  body.  Amcebse  are  rarely  free  from 
such  "symbiotic"  bacteria,  and  it  is  probable 
that  the  digestive  processes  of  all  herbivorous 
animals  are  assisted  by  these  symbiotic  bacteria. 
SYMBIOTIC  ALG^E  — The  most  remarkable  case 
of  symbiosis  in  animals  is  one  due  to  the  occur- 
rence of  a  minute  unicellular  green  or  yellow 
alga  in  the  lower  invertebrates.  It  has  long  been 
known  that  many  green  animals  occur,  and  that 
their  green  colour  is  due  either  to  plant  green  or 
to  a  closely  similar  substance.  There  are  green  as 
well  as  colourless  Amcebse,  green  and  colourless 


SOCIETIES   AND   ASSOCIATIONS    157 

Paramecia,  green  and  colourless  Hydra,  green 
and  colourless  sponges  of  the  same  species. 
Certain  of  the  Acoelomate  "worms"  or  Pla- 
narians  are  green — for  example,  Convoluta  ros- 
coffensis,  a  marine  species  that  is  never  colourless. 
Its  ally  Convoluta  paradoxa  is  also  green,  but  the 
colour  is  masked  by  a  yellow-brown  pigment. 
Many  sea-anemones  are  green  or  green  overlaid 
by  brown.  Many  corals  are  green  or  brown. 
In  fact,  from  the  Protozoa  up  to  the  Mollusca, 
there  are  species  in  each  family  which  contrast 
with  their  fellows  in  occurring  either  in  two  colour 
forms,  one  of  which  is  green  or  brown,  or  in  a 
purely  green  variety. 

For  a  long  time  the  nature  of  this  colouring 
matter  was  disputed,  and  even  now  it  is  not 
certainly  ascertained  in  a  few  cases.  All  those 
instances  we  have  mentioned  have,  however, 
been  shown  to  contain  algae,  either  of  the  green 
or  brown  order,  or  more  rarely  (in  sponges  and 
star-fish)  of  the  red  order.  The  peculiar  colour 
variety  owes  its  distinctive  colour  to  the  in- 
fection of  its  tissues  by  a  plant. 

THE  CASE  OF  CONVOLUTA. — The  most  clearly 
ascertained  case  is  that  of  Convoluta  roscoffen- 
sis  (Fig.  15),  a  small  planar ian  about  one-sixth 
of  an  inch  in  length.  When  walking  over  the 
sandy  beach  of  Brittany  at  low-tide,  an  irregular 
green  scum  may  be  noticed  at  about  high-tide 


158  THE  ANIMAL  WORLD 

level,  apparently  pouring  out  of  the  sand  and 
extending  sea-wards  for  a  few  feet  in  the  form 
of  splashes  or  zones  of  a  deep  green  tint.  On 
treading  near  the  patches  the  green  scum  is 
seen  to  disappear  into  the  sand  and  to  rise 
again  to  the  surface  when  the  disturbance  of 
footsteps  has  passed  away.  When  the  tide 
rises  and  begins  to  lap  the  patch  it  sinks  rapidly 
out  of  sight,  and  remains  below  the  sand  until  the 
next  ebb-tide,  when  it  emerges  with  the  water 
that  wells  up  out  of  the  beach.  If  the  place  is 
marked  and  the  spot  revisited  next  day  it  will  be 
found  occupied  as  before  by  a  green  scum.  In 
fact,  year  after  year  these  remarkable  patches 
with  few  exceptions  maintain  their  positions. 

Examination  of  the  scum  shows  that  the 
green  colour  is  due  to  a  vast  number  of  minute 
green  planarian  worms  which  have  come  to 
occupy  a  zone  on  the  beach  which  is  exposed 
for  the  longest  time  to  the  sun  compatible  with 
not  being  desiccated.  Such  vast  gatherings 
of  green  planarians  are,  at  present,  known  only 
in  Brittany,  part  of  Normandy,  and  the  east 
coast  of  Africa;  and  they  are  remarkable,  not 
only  for  their  colour,  but  for  the  discrepancy 
between  their  behaviour  and  that  of  their  allies. 
Planarians  are  soft-bodied  voracious  animals, 
living  concealed  during  the  day  and  only  emer- 
ging at  night.  Like  most  .other  carnivorous 


SOCIETIES  AND  ASSOCIATIONS   159 

animals,  they  are  not  social,  though  a  few  species 
aggregate  under  the  same  stone  during  their 
inactive  phase.  The  green  Convoluta,  how- 
ever, makes  no  effort  to  conceal  itself — on  the 
contrary,  it  lies  in  as  exposed  and  conspicuous 
a  position  as  could  well  be  chosen.  It  associates 
in  vast  colonies,  and  does  not  appear  to  live  by 
taking  in  any  solid  food.  Though  it  can  be  made 
to  ingest  all  manner  of  fine  particles,  Convo- 
luta does  not  do  so  in  a  state  of  nature. 

This  aberrant  behaviour  is  explained  when 
the  nature  of  the  green  colour  is  fully  investi- 
gated; At  first  sight  the  animal  appears  to 
be  thickly  sown  with  oval  green  spots  several 
hundreds,  or  even  thousands,  in  each  planarian. 
These  spots  are  quite  unlike  any  algse,  and  it 
is  only  after  prolonged  experiments  that  they 
have  proved  to  be  green  algse,  highly  modified 
in  consequence  of  their  association  with  the 
animal.  In  a  free  state  they  exist  in  the  neigh- 
bouring ocean  as  minute  ciliated  "Flagellates." 
Some  flagellates,  such  as  Euglena  (Fig.  2,  p.  19) 
are  a  common  source  of  green  scum  in  roads 
and  farmyards,  and,  like  their  marine  allies, 
they  are  constantly  tried  by  the  scarcity  of 
nitrogen.  In  fact,  as  farmers  know,  the  whole 
question  of  crops  may  be  said  to  be,  how,  at 
least  cost,  to  increase  the  amount  of  available 
nitrogen  in  the  soil,  and  the  scarcity  of  nitrogen 


160 


THE  ANIMAL  WORLD 


in  the  sea  is  greater  than  on  land  in  consequence 
of  there  being  no  manure,  and  therefore  fewer 
bacteria  to  form  nitrates.  Any  means,  there- 


Fig.  15. 


Fig.  16. 


Fig.  15.  —  Convoluta  roscoffensis:  the  green  social  Planarian 
of  Brittany.  (  X  50.) 

This  animal  contains  remarkable  green  cells  (see  fig.)» 
to  which  its  colour  is  due.  These  cells  are  represented 
by  the  minute  dots  arranged  in  rows  down  the  body. 
Near  the  front  end  and  in  the  middle  line  is  the  "otolith" 
(O),  an  organ  for  appreciating  disturbance.  On  either 
side  of  this  is  a  rudimentary  eye-spot  (E),  by  which 
light  and  shadow  are  felt.  The  other  dark  spots  are 
local  accumulations  of  used-up  green  cells. 

Fig.  16.  —  One  of  the  green  cells  or  algae  which  infect  Con- 
voluta. The  four  flagella  or  whip-like  filaments  enable 
these  microscopic  organisms  to  swim  actively  for  a  time. 
Upon  entering  the  body  of  Convoluta  they  are  lost. 
(  X  1000.) 


SOCIETIES  AND  ASSOCIATIONS    161 

fore,  that  will  give  algse  and  diatoms  supplies 
of  nitrogen,  are  readily  employed.  Now  each 
Convoluta  lays  every  fortnight  one  or  more 
batches  of  eggs  in  clutches  of  six  to  eight,  each 
clutch  being  placed  in  a  common  envelope 
and  sunk  in  the  sand.  The  act  of  egg-laying 
is  accompanied  by  more  or  less  rupture  of  the 
body-wall  and  the  discharge  of  a  certain  amount 
of  tissue.  On  this  debris  and  on  the  egg-mem- 
branes, bacteria  assemble  and  multiply.  Here 
on  the  site  of  the  patch  where  thousands  of 
Convoluta  have  laid  eggs  month  after  month 
there  is  an  abundant  supply  of  food  material 
for  bacteria,  and  consequently  a  great  local 
accumulation  of  nitrogen.  Here,  then,  colonies 
of  the  algae  settle  down  from  the  sea-water 
and  multiply,  often  indeed  within  the  egg- 
capsules  of  Convoluta. 

In  a  few  days  (five  or  six)  after  the  eggs  are 
laid,  Convoluta  hatches  as  a  minute,  colourless 
miniature  of  its  parent.  Provided  with  a  large 
ciliated  mouth  it  scours  the  egg-capsule  and 
its  neighbourhood  for  suitable  food.  At  this 
stage  Convoluta  will  ingest  almost  any  particles 
small  enough  to  enter  its  relatively  capacious 
but  actually  small  mouth.  We  here  strike 
upon  the  fact  referred  to  on  another  page,  that 
often  only  one  food  is  suitable  to  start  the 
growth  of  a  new-born  infant.  Just  as  milk  is 


162  THE  ANIMAL  WORLD 

the  only  food  for  human  infants,  so  Convoluta- 
infants  require  a  special  food — in  this  case  the 
green  flagellates,  whatever  else  the  young 
Convohda  laps  up,  without  the  algae  its  growth 
does  not  proceed  and  it  soon  dies.  But,  as  we 
have  seen,  an  abundant  store  of  this  flagellate 
exists  close  at  hand,  and  in  the  course  of  a  few 
hours  after  birth  every  infant  Convoluta  obtains 
a  supply  of  the  green  algae  that  it  needs.  As 
soon  as  this  supply  is  obtained  Convoluta  ceases 
to  absorb  solid  food. 

This  remarkable  result  is  due  to  far-reaching 
changes  in  the  green  cells  that  are  ingested. 
Instead  of  undergoing  rapid  digestion  they 
appear  to  flourish.  They  divide  and  re-divide 
and  become  spaced  out  as  green  dots  through 
the  soft  integument  of  the  animal.  With  their 
entrance  and  dissemination,  Convoluta  assumes 
the  peculiar  habit  of  its  kind,  acquiring  the 
social  faculty,  the  sunny  position  and  the  still- 
ness that  so  markedly  differentiate  it  from  its 
allies.  The  key  to  these  changes  lies  in  the 
needs  of  the  green  cells.  Like  all  plants  these 
require  light  and  carbon  dioxide  if  they  are  to 
manufacture  and  accumulate  stores  of  reserve 
material.  These  and  other  favourable  conditions 
are  ensured  by  the  behaviour  of  Convoluta. 
By  their  sociable  disposition  these  animals 
create  accumulations  of  carbon  dioxide;  in 


SOCIETIES  AND  ASSOCIATIONS    163 

their  soft  bodies  are  bacteria  and  stores  of  nitro- 
gen; by  their  sunlit  attitude  on  warm  coasts 
long  spells  of  isolation  are  ensured  (only  one 
and  that  a  rare  enemy  attacks  Convoluta).  Little 
wonder,  therefore,  that  this  green  flagellate 
alga  finds  in  the  body  of  Convoluta  a  safe  and 
favourable  harbour  of  refuge.  If  that  were  all, 
however,  the  case  would  be  one  of  parasitism 
and  not  of  symbiosis,  for  by  a  symbiosis  a  mutual 
benefit  is  signified.  Convoluta,  however,  derives 
several  advantages  from  the  association.  First 
of  all,  as  we  have  seen,  it  derives  its  first  food 
from  this  alga;  but  it  does  this  in  a  subtle  way. 
Occasionally  engulfing  and  digesting  the  whole 
green  cell,  Convoluta  feeds  chiefly  by  absorbing 
the  reserve-material  (possibly  sugar)  created 
by  the  alga  together  with  pigments  of  the  green 
cell  itself.  This  does  not  deplete  its  stores 
since  a  wide  margin  of  untouched  cells  are  left 
over  to  make  good  by  division  the  loss  of  those 
that  are  eaten.  Convoluta  has  now  created  an 
original  style  of  living  and  is  independent  of 
its  environment.  I 

Such  in  brief  is  the  story  of  Convoluta,  and  it 
illustrates  an  extreme  case  of  association,  for 
without  its  associated  green  cells  Convoluta 
dies.  The  green  cells  are  probably  capable  of 
living  independently  of  Convoluta,  but  flourish 
best  in  association  with  it. 


164  THE  ANIMAL  WORLD 

SYMBIOTIC  ALG^S  IN  THE  LOWER  ANIMALS. — 
The  other  cases  of  algse  living  on  or  in  vari- 
ous animals  represent  earlier  phases  in  this 
dependence.  Protozoa,  anemones  and  corals 
can  exist  without  their  so-called  commensal 
algse  but  not  so  well  as  when  infected  with 
them,  and  like  all  biological  advantages  it  is 
not  the  absolute  but  the  relative  that  matters; 
not  the  capacity  to  live  but  the  capacity  to  live 
better,  to  grow  quickly  and  strongly,  to  gain 
the  early  supplies  of  food,  the  earlier  nesting 
places,  and  so  on.  So  it  is  with  these  coloured 
animals.  They  gain  advantage  over  their 
colourless  brethren  by  the  forcing  food  supplied 
through  the  agency  of  these  algal  cells.  They 
grow  more  quickly,  divide  more  rapidly  and 
get  rid  of  their  nitrogen  waste  products  more 
effectually.  They  possess  in  these  coloured 
cells  a  multiplying  reserve  food  upon  which 
they  can  depend  in  time  of  scarcity. 

This  relation  of  animals  to  plants  living  with- 
in them  in  varying  degrees  of  intimacy,  has 
only  recently  been  appreciated,  but  is  becoming 
more  and  more  widely  recognized.  During 
the  last  year,  for  example,  the  discovery  has 
been  made  that  at  least  one  order  of  insects, 
the  Hemiptera,  possess  living  in  their  green 
or  red  coloured  cells,  cultures  of  yeasts  which 
appear  to  be  transmitted  from  parent  to  off- 


SOCIETIES  AND  ASSOCIATIONS   165 

spring  (e.  g.  Aphides);  and  it  is  probable  that 
further  investigations  which  are  now  in  progress 
will  reveal  the  essentially  compound  or  com- 
posite nature  of  many  other  animals  and  plants 
that  are  now  regarded  as  purely  individuals. 
The  idea  of  symbiosis  is  rapidly  becoming  a 
dominant  one. 

COMMENSALISM. — The  commensal  tie,  that 
is,  the  bond  of  a  common  table,  is  at  the  bot- 
tom of  many  associations.  The  feudal  system, 
the  servant  system,  are  types  of  these,  and 
just  as  the  bond  which  unites  groups  of  men 
is  various — a  totem,  kinship  through  father 
or  through  mother,  payment  in  kind  or  in  cur- 
rency— so  animal  associations  are  based  upon 
many  forms  of  relationship,  partnership  or 
exchange.  As  we  have  seen,  the  lower  animals 
form  associations  of  their  own  kinds  by  budding, 
and  associations  of  a  different  order  by  symbiosis. 

1.  Inquilines. — Under  this  head  are  included 
those  apparently  casual  but  often  important 
associations  which  are  formed  between  alien 
animals.  The  most  important  of  these  is  due 
to  the  taming  and  domestication  of  wild  ani- 
mals by  man.  This  art  has  had  a  decisive  effect 
on  man's  history  and  to  it  more  than  to  any 
other  simple  factor  the  origin  of  civilization  is 
due.  Animals  have  the  enormous  advantage 
over  other  forms  of  possession  of  being  repro- 


166  THE  ANIMAL  WORLD 

ductive.  Many  of  our  words,  such  as  "capital" 
and  "pecuniary,"  remind  us  of  pre-currency 
ages,  when  animals  were  a  medium  of  exchange, 
a  reward  for  labour  and  for  valour,  a  means 
of  acquiring  retainers  and  of  tilling  the  ground, 
and  constituted  the  earliest  form  of  property 
(Maine,  Early  History  of  Institutions). 

HERMIT-CRABS  AND  ANEMONES. — Amongst 
animals,  associations  of  unrelated  species  begun 
in  a  casual  fashion  rarely  assume  great  im- 
portance owing  to  the  lack  of  that  character- 
istically human  faculty  of  improving  upon 
experience.  Our  common  hermit-crabs  require 
the  shelter  of  a  gasteropod  shell  and  are  never 
found  except  in  such  shells.  One  species,  how- 
ever, is  found  bearing  not  only  a  shell  but  a 
shell  crowned  by  an  anemone  which  does  not 
occur  elsewhere.  Another  is  encrusted  with 
a  sponge  which  gradually  dissolves  away  the 
calcareous  shell,  and  this  species  usually  har- 
bours a  Nereid  worm  which  is  only  found  in 
this  situation.  How  far  these  are  true  com- 
mensals living  at  the  same  table  provided  by 
the  sea  is  at  present  doubtful.  The  advantages 
of  their  mutual  association  seem  to  lie  in  the 
fact  that  whilst  hermit-crabs  are  greedily  sought 
after  by  many  fish,  the  stinging  anemone  and 
the  spiny  sponges  are  avoided,  and  therefore 
the  edible  hermit  is  protected  by  the  presence 


SOCIETIES  AND  ASSOCIATIONS    167 

of  its  inedible  associates.  The  worm,  move- 
over,  is  also  a  bonne-bouche  for  most  fish,  and 
therefore  gains  by  lurking  in  the  coils  of  such 
shells.  More  striking  instances  of  such  coali- 
tions are  seen  in  the  fish  Fierasfef,  which  hides 
itself  in  the  large  sea-cucumbers  or  Holothurians 
of  the  warm  seas  ;  in  the  coral  reef  fish  and 
prawn  that  dive  into  the  interior  of  large  sea- 
anemones  at  the  approach  of  an  enemy.  Even 
on  our  own  coasts  shoals  of  fry  often  accom- 
pany the  larger  medusae  that  drift  north  and 
westwards  in  summer,  and  at  the  approach 
of  a  shadow  or  disturbance,  snuggle  under 
the  protecting  disc  and  lie  protected  by  the 
stinging  tentacles.  Certain  Crustacea,  notably 
the  reddish  Hyperia  galba,  also  associate  with 
the  medusa  Aurelia. 

ANTS  AND  THEIR  GUESTS. — Passing  from 
this  association  of  unrelated  animals  for  pro- 
tective purposes,  we  may  next  notice  the  way 
in  which  certain  animals  employ  or  permit  the 
existence  of  others  in  their  family  life.  In  an- 
other chapter  the  care  of  the  young  is  separately 
considered  and  need  only  be  referred  to  here 
in  order  to  point  out  that  these  brooding  and 
social  species  often  adopt  members  of  wholly 
unrelated  families.  Some  even  go  further  than 
this  and  make  expeditions  for  the  purpose  of 
capturing  slaves  from  an  allied  species.  Thus 


168  THE   ANIMAL  WORLD 

there  are  slave-making  ants  in  America  and 
Europe  which  raid  the  nests  of  other  species, 
carry  off  the  larvae  and  pupse  and  rear  them 
in  their  own  nests.  Some  ants  import  the  eggs 
of  aphides  and  rear  them  in  order  to  possess 
a  store  of  sugar-secreting  "cows."  More 
commonly  species  of  beetles  and  Hemiptera 
are  introduced  by  workers  for  the  purpose  of 
providing  the  inmates  with  a  supply  of  an 
ethereal  drink  which  such  inquilines  secrete 
at  certain  points  of  their  bodies.  Many  of 
these  inquilines  are  now  only  found  in  ants' 
nests  and  twelve  hundred  different  kinds  of 
such  guests  are  already  known,  whilst  at  least 
a  hundred  are  known  from  the  nests  of  the  ter- 
mites or  so-called  white  ants.  Some  of  these 
guests,  such  as  the  beetles,  are  by  no  means 
innocuous,  since  they  feed  upon  the  eggs  or 
larvae  of  the  ants  and  divert  the  workers  from 
their  proper  duties. 

THE  CUCKOO  HABIT. — Closely  related  with 
this  adoption  of  diverse  animals  by  ants  and 
termites  is  the  cuckoo  habit  of  foisting  the 
care  of  offspring  upon  a  foster-parent.  It  is 
known  that  the  English  cuckoo  lays  her  egg 
down,  and  after  picking  it  up  with  her  bill  in- 
serts it  in  the  nest.  The  foster-parents  in  this 
country  include  a  wide  range  of  birds — hedge 
sparrows,  pipits,  warblers,  and  others,  and 


SOCIETIES  AND  ASSOCIATIONS    169 

in  some  cases  it  is  asserted  that  the  foster-birds 
even  neglect  their  own  young  in  attending  to 
the  more  vigorous  demonstrations  of  the  cuckoo 
nestling.  Even  after  the  young  cuckoo  is  fledged 
and  grown,  its  connection  with  the  nest  in  which 
it  was  reared  is  probably  not  broken.  The  pe- 
culiarly variable  colouring  of  cuckoo  eggs  which 
often  renders  them  almost  indistinguishable 
from  those  of  their  foster-parents  is  only  in- 
telligible on  the  hypothesis  that  the  hen-bird 
which  has  laid  in  a  given  nest  comes  back  at 
the  next  season  to  the  same  nest  and  if  this 
is  occupied  she  drops  her  egg  in  another.  This 
would  at  least  explain  why  cuckoo's  eggs  some- 
times resemble  and  sometimes  contrast  with 
those  of  the  foster-parents. 

CUCKOO-BEES. — Amongst  bees  a  similar 
cuckoo  habit  is  well  known.  Such  cuckoo-bees 
are,  however,  even  more  remarkable  than  birds, 
since  they  have  lost  the  pollen-collecting  ap- 
paratus of  their  allies.  Cases  like  these  lead 
on  to  parasitism  in  which  the  young  of  one 
species  finds  food  and  shelter  in  or  on  the  bodies 
of  their  host.  This  habit,  however,  is  too  large 
a  subject  to  be  dealt  with  here,  and  a  short 
account  of  it  is  therefore  given  in  Chapter  VIII. 

GREGARIOUS  HABITS,  MIGRATORY  HABITS. — 
The  last  form  of  association  we  can  notice  is  that 
of  flocking:  the  gregarious  habit  adopted  by 


170  THE  ANIMAL  WORLD 

members  of  the  same  species.  This  habit  is  most 
developed  in  those  animals  which  have  highly 
organized  and  constantly  exercised  movements, 
but  it  is  also  seen  in  the  young  stages  of  many 
species  which  are  solitary  when  adult.  Amongst 
insects,  examples  of  both  these  forms  of  herding 
occur.  Many  insects  migrate  at  regular  or 
irregular  intervals,  and  at  such  times  assemble 
like  birds  in  vast  flocks.  The  migratory  instinct 
in  some  locusts  arises  in  the  young  before  the 
wings  are  attained,  and  vast  hordes  of  a  single 
species  may  migrate  on  foot  over  land  and  river; 
others  only  flock  after  the  wings  are  fully  grown. 
Some  butterflies  are  migratory,  and  vast  flocks 
have  been  described  crossing  from  India  to  Ceylon. 
Many  species  of  birds  are  social  at  the  end  of  the 
breeding  season  on  the  southward  migration, 
during  the  summer  (puffin,  guillemots,  terns, 
gulls),  or  during  the  winter  (plovers,  rooks,  star- 
lings). Sometimes  the  vast  flocks  of  rooks  and 
starlings  are  composed  of  non-resident  birds 
driven  by  severer  conditions  to  a  milder  climate, 
at  other  times  they  are  more  probably  residents 
drawn  together  by  abundance  of  food:  for  in- 
stance, by  a  plague  of  wire-worms  (larvae  of 
click-beetles)  or  of  leather- jackets  (larvae  of 
crane-flies).  On  these,  as  on  so  many  other 
points  of  natural  history,  there  is  still  much  to 
be  learnt.  The  most  impressive  features  of 


SOCIETIES  AND  ASSOCIATIONS    171 

these  flocks  is  the  dexterity  with  which  they 
perform  complex  evolutions,  moving  in  unison 
and  with  a  wonderful  nicety  of  judgment. 

A  similar  gregarious  habit  is  seen  amongst 
certain  fish;  usually  it  is  shown  either  at  the 
breeding  season  or  by  the  young.  Herring  shoal 
on  certain  grounds  for  the  purpose  of  depositing 
their  eggs  on  weeds  and  stones.  Pilchards  on 
the  west  of  France  also  shoal  at  certain  seasons 
in  the  young  "sardine"  stage.  Mackerel  appear 
on  our  coasts  in  spring  and  summer,  following 
the  abundance  of  plankton  or  minute  drifting 
life.  Many  other  fish  congregate  during  the 
breeding  season  on  special  grounds:  for  example, 
in  the  North  Sea  and  on  the  Newfoundland 
banks.  Their  fry  also  shoal  and  exhibit  those 
superbly  timed  movements  which  are  seen 
amongst  birds.  Amongst  some  animals  flocks 
are  formed  only  by  members  of  one  sex.  Thus 
a  cloud  of  male  Bibio  (a  two-winged  fly  in  which 
there  is  marked  difference  between  male  and 
female)  is  often  seen  making  rapid  flights  back 
and  forth  over  a  stretch  of  water,  turning  with 
the  greatest  ease  at  each  end  of  its  pendulum-like 
swing.  Male  gnats  are  often  so  numerous  as  to 
form  a  haze  that  floats  over  the  country-side. 

Perhaps  the  most  impressive  of  all  flocking 
movements  is  the  swarming  of  hive-bees  and 
ants.  In  the  case  of  bees  there  are  two  distinct 


172  THE  ANIMAL  WORLD 

emigrations:  the  nuptial  flight,  when  the  queen 
is  followed  by  a  swarm  of  drones;  and  the  swarm, 
when  the  queen,  followed  by  a  host  of  workers, 
leaves  the  hive  at  its  height  of  prosperity  in 
order  to  found  a  new  home.  In  each  of  these 
wonderful  manoeuvres  the  behaviour  of  the  bees 
is  profoundly  different  from  the  normal.  The 
virgin  queen,  who  has  previously  never  crossed 
the  door  of  the  hive,  flies  far  up  into  the  sky 
and  the  drones  that  have  spent  their  lives  in 
lapping  honey  from  the  combs  suddenly  awaken 
to  a  true  sense  of  their  powers,  and,  gifted  with  a 
prodigious  capacity  for  catching  the  magnetic 
queen-perfume  by  the  thousands  of  sensory  pits 
on  their  antennae,  pour  forth  in  a  great  ascending 
stream  until  in  the  distant  blue  one  victoriously 
mates  with  the  elusive  queen  and  perishes  in  the 
act. 

In  swarming,  on  the  other  hand,  the  workers 
leave  their  accustomed  routine,  a  few  advance 
as  scouts  looking  for  a  suitable  tree  as  resting- 
place,  whilst  the  remainder  to  the  number  of  tens 
of  thousands  follow  the  queen  in  a  dazed  condi- 
tion, settling  in  clouds  on  the  selected  spot,  and 
may  then  be  handled  with  an  ease  impossible  at 
other  times. 

Ants  on  an  appointed  day,  generally  a  still,  hot 
August  day,  may  be  seen  issuing  in  a  jet  from 
holes  in  the  ground  under  which  their  nests  lie. 


THE  CARE  OF  THE  YOUNG  173 

These  living  streams  are  the  winged  males  and 
females  which  have  never  flown  before,  and  which 
now  make  their  nuptial  flight,  leaving  the  workers 
behind.  Their  feeble  flight  is  soon  over;  but 
for  a  time  the  air  is  thick  with  drifting  flakes. 
Mating  is  accomplished  in  mid-air,  and  then  the 
living  shower  descends  over  the  country-side, 
pours  down  our  chimneys,  drifts  over  the  roads 
and  is  devoured  by  all  manner  of  hungry  birds 
and  beasts. 


CHAPTER  VIII 

THE    CARE    OF    THE    YOUNG 

ADOLESCENCE. — Animals,  like  plants,  show  the 
most  varied  devices  for  ensuring  the  welfare  of 
the  race.  In  a  growing  and  developing  animal 
the  issue  of  life  appears  purely  personal  and  self 
centred.  Its  internal  structure  and  its  inner 
world  are  adapted  to  evoke  the  unfolding  of 
its  capacity  for  filling  a  niche  in  the  vast  fabric 
of  existence.  It  goes  from  strength  to  strength, 
from  simplicity  to  complexity,  from  response  to 
response,  and  ultimately  becomes  an  organic 
fabric  and  constellation  moving  with  definite 
impulses  in  a  corner  of  the  world:  and  then, 
when  its  life  has  settled  down  into  a  routine  of 
rhythmical  action  and  repose,  its  behaviour 


174 


THE  ANIMAL  WORLD 


suddenly  changes  and  its  whole  reserve  of  energy 
and  of  material  is  drawn  upon  for  a  cause  not  its 
own  but  that  of  the  race  to  which  it  belongs. 
With  the  advent  of  this  racial  impulse  its  ap- 


Fig.  17.  —  A  Polyzoon  or  Sea-mat  (Flustra). 

A.  Portion  of  a  colony  of  Flustra  such  as  is  often  thrown  up 

on  the  beach.     (  X  1.) 

B.  A   portion  of  the   same   highly  magnified   to   show  the 

polypides  or  individual  chambers  of  which  it  is  composed. 
Each  polypide  has  a  lid,  which  in  some  is  closed,  in 
others  open. 

C.  Shows  the  free-swimming  larva  of  these  Polyzoa  (  X  500). 

It  is  a  minute,  opaque,  conical  organism,  capable  of 
swimming  by  the  aid  of  the  long  cilia  round  its  lower 
border.  The  body  is  enclosed  in  a  delicate  shell,  and 
ends  in  a  sensitive  apex.  (After  Harmer.) 

pearance  may  attain  a  more  splendid  aspect;  a 
new  fighting  response  may  arise  at  each  encounter 
with  its  fellows.  If  vocal,  its  voice  deepens;  if 
coloured  its  coat  brightens.  It  may  display  its 
strength  and  its  finery  before  members  of  the 


THE  CARE  OF  THE  YOUNG   175 

opposite  sex  and  there  may  ensue  a  selection  of 
the  stronger  or  more  attractive  matesr  who  thus 
obtain  a  lead  over  their  feebler  and  less  fortunate 
fellows.  But  in  this  momentous  epoch  no  rigid 
line  of  conduct  can  be  recognized.  Animal 
behaviour  at  the  mating  season  offers  wide  con- 
trasts, not  only  between  members  of  different 
orders  and  genera,  but  between  individuals  of 
the  two  sexes.  The  male  may  attain  a  size  and 
exhibit  an  exuberance  of  colour,  he  may  develop 
an  activity  and  a  series  of  boldily  appendages 
that  contrast  strangely  with  the  aspect  and  be- 
haviour of  his  desired  mate.  On  the  other  hand, 
he  may  be  but  a  mere  forgotten  incident  in  the 
long  life  and  dominant  activity  of  his  widow,  for 
in  such  cases  he  dies  from  natural  causes  of  which 
his  mate  must  be  reckoned  as  the  chief. 

MODIFICATIONS  OF  SEX. — The  assumption  of 
sex  by  animals  is,  however,  complicated  by  many 
arresting  factors.  Neuters,  that  is,  impotent 
individuals,  are  known  amongst  social  animals 
and  are  familiar  to  us  as  "workers"  amongst 
ants,  bees  and  wasps.  Occasionally,  however, 
these  workers  become  fertile  and  then  lay  a  few 
eggs,  showing  that  they  are  arrested  females. 
Amongst  Termites,  the  so-called  white  ants  of 
Africa  and  southern  Europe,  there  are  both  work- 
ers and  soldiers  which  are,  strange  as  it  may 
seem,  arrested  males  and  females.  Nor  does 


176  THE   ANIMAL  WORLD 

the  variety  of  nature  stop  here,  for  there  are 
vast  numbers  of  animals,  for  instance,  green  fly 
or  Aphides,  which  for  months  together  and  in 
some  cases  for  years  in  succession,  produce 
young  without  the  appearance  of  any  males. 
Lastly,  there  are  some  remarkable  parasitic 
Crustacea  and  certain  of  the  lower  worms  which 
attain  two  periods  of  maturity  under  diverse 
conditions  of  bodily  development.  Early  in 
life  and  at  a  stage  of  development  that  seems  to 
promise  a  high  grade  of  organization  their  growth 
is  arrested  and  they  become  males.  Then  degen- 
eration ensues,  the  plan  of  organization  is  lowered 
and  though  growing  in  bulk  they  become  less 
and  less  highly  organized  and  in  this  state  be- 
come female. 

'  BIRTH  OF  LARVAE  IN  THE  SEA. — The  eggs  of 
most  marine  animals  are  discharged  freely  into 
the  sea;  in  such  cases  the  young  swim  freely  or 
drift  far  from  their  birthplace.  For  example,  the 
sponges,  hydroids,  corals,  sea-mats,  all  the  worms, 
all  the  molluscs,  most  of  the  barnacles,  prawns 
and  crabs  and  a  large  number  of  fish,  cast  their 
young  upon  the  waters,  leaving  them  to  feed  for 
themselves  and  to  wander  whither  the  currents 
carry  them.  The  majority  of  such  animals  are 
fixed  or  slowly-moving  creatures,  and  it  is  an 
axiom  in  zoology  that  sessile  parents  have  active 
young.  Consequently  the  structure  of  the  young 


THE  CARE  OF  THE  YOUNG   177 

stage  is  adapted  to  an  entirely  different  life  from 
the  one  that  it  will  adopt  later  on.  It  swims  or 
drifts  freely,  now  nearer  the  surface,  now  at  a 
deeper  level  in  order  to  keep  in  touch  with  the 
oscillations  of  the  minute  animal  and  plant  life 
upon  which  it  feeds.  Frequently  this  food  of 
the  young  is  again  different  from  that  of  its 
parents  and  new  means  of  catching,  digesting 
and  absorbing  it  are  required.  The  result  of 
these  adaptations  is  a  creature  so  different  (apart 
from  size)  from  its  parents  as  to  deceive  even  the 
elect;  and  it  is  only  by  actually  breeding  the 
barnacle,  the  oyster  and  the  sea-mat,  that  the 
connection  between  their  active  free-swimming 
young  and  the  sedentary  mother  can  be  realized. 
Such  animals  are  said  to  have  a  larval  stage.  The 
organs  of  the  larvae  or  free-swimming  young  are 
not  converted  directly  into  those  of  the  adult 
stage.  They  are  needed  for  one  mode  of  life 
and  conform  thereto,  but  they  are  not  suitable 
for  the  later  habits.  Hence  a  more  or  less  violent 
change  is  effected  and  this  change  is  called  meta- 
morphosis. Metamorphosis  is  accompanied  by 
the  assumption  of  the  adult  habits.  The  active 
swimmer  touches  a  rock,  floating  log,  or  ship's 
hulk  and  after  fixing  itself  becomes  transformed 
into  a  totally  diverse  sort  of  creature  with  seden- 
tary habits  and  little  intercourse  with  the  moving 
world  around  it.  Instead  of  drawing  itself 


178 


THE  ANIMAL  WORLD 


through  the  water,  it  now  draws  water  through 
itself. 

DANGERS  OF  LARVAL  LIFE. — The  fate  of  the 
majority  of  larvae  is  by  no  means  always  the  con- 
summation we  have  sketched.  The  perils  of  the 


Fig.  18.  —  A.  Goose-barnacle  (Lopas),  such  as  is  found  ad- 
hering to  ships'  bottoms  by  its  long  stalk.  The  body 
is  enclosed  in  a  shell,  from  which  the  feet  project.  (  X  1.) 

B.  Free-swimming  larva  of  the  same,  showing  the  first 
three  pairs  of  limbs  which  are  used  for  swimming  and 
for  collecting  food.  The  long  processes  (near  B)  help 
to  buoy  up  the  animal.  (  X  100.) 

uncharted  ocean  are  on  every  side,  and  even  if 
the  more  serious  dangers  of  predaceous  foes  and 
of  starvation  be  evaded,  there  is  still  the  difficulty 
of  finding  the  haven  where  anchorage  may  be 
dropped  and  adult  life  assumed.  Experience  of 
breeders  shows  how  excessively  delicate  are 
most  of  these  larvse  and  how  meticulous  is  their 


THE  CARE  OF  THE  YOUNG   179 

choice  of  food,  choice  of  water  and  of  other  re- 
quirements. Probably  only  one  oyster  in  a 
million  comes  to  anchor,  only  one  codfish  in  five 
millions  completes  its  maturity.  The  larvae 
only  live  by  a  hair,  and  the  least  change  is  fatal 
to  myriads  of  them.  Added  to  these  intrinsic 
dangers  as  we  may  call  them,  are  the  foes  which 
pursue,  harass  and  devour  such  tender  morsels. 
Young  fish  are  killed  by  minute  Copepods  smaller 
than  themselves,  and  are  engulfed  by  larger  fish, 
birds  and  Crustacea. 

SIZE  OF  FAMILIES. — In  order  to  compensate 
for  this  prodigious  infant  mortality,  animals  have 
to  produce  vast  numbers  of  eggs,  and  the  drain 
of  such  numbers  upon  the  resources  of  the  mother 
is  very  great.  A  turbot  may  spawn  nine  millions 
in  a  season,  a  cod  five  millions,  a  flounder  one 
million.  The  result  of  this  vast  output  of  matter 
and  energy  is  a  depletion  of  the  parental  stores. 
The  flesh  of  such  fish  assumes  a  watery,  flabby 
texture,  the  body  is  thin  and  pale  and  the  weight 
much  diminished.  Such  "spent  fish"  take  a 
considerable  time  before  regaining  their  prime 
condition. 

MIGRATION  TO  BREEDING-GROUNDS. — In  rare 
cases  the  parents  of  fish-larvse  and  of  worm-lar- 
vae make  migrations  to  a  breeding-ground.  Just 
as  migration  in  birds  is  a  movement  from  the 
feeding-ground  to  the  nesting-ground,  so  there 


180  THE  ANIMAL  WORLD 

are  many  active  marine  animals  which  pass  the 
year,  in  part  in  one  water  to  feed,  in  part  in 
another  to  breed.  Such  movements  are  heralded 
by  a  change  of  aspect  in  the  migrants.  Eels 
assume  a  shining  dress  and  larger  eyes.  Many 
sea-worms  or  Annelids  develop  paddling  feet 
instead  of  crawling  ones,  their  colour  changes, 
their  eyes  enlarge,  and  the  bristles  alter  in  shape 
and  in  size.  In  this  livery,  an  outward  expression 
of  a  change  of  inner  state,  the  migrants  embark 
on  their  voyage;  some,  such  as  eels,  hurtling  in 
great  companies  with  the  spate  down  to  the  sea 
and  far  out  from  the  coast  to  the  deep  waters; 
others,  such  as  salmon  and  smelt,  make  for  the 
rivers  and  ascend  over  fall  and  weir  to  the 
favourite  bank. 

BREEDING  HABITS  OF  PALOLO-WORM. — The 
most  remarkable  of  these  breeding  migrations  is 
that  of  the  sea- worms.  The  Nereis,  for  example, 
that  lives  in  burrows  either  between  tide  marks 
or  in  company  with  a  hermit-crab  (p.  166),  for- 
sakes the  burrow  and  striking  out  with  the  new 
oars,  directed  by  the  enlarged  eyes  and  guided 
by  the  increased  sensitiveness,  launches  upon 
the  ocean.  There,  sometimes  in  the  heat  of  the 
day  or  in  the  darkness  of  night,  they  row,  spurt- 
ing out  their  eggs  with  every  stroke  and  even 
dislocating  their  bodies  into  fragments  with  the 
fury  of  their  movements.  On  the  coast  of  Flor- 


THE  CARE  OF  THE  YOUNG   181 

ida,  Malaya  and  of  Polynesia,  the  coral  rock  is 
full  of  burrowing  Annelids,  and  amongst  these 
are  some  species  of  Palolo,  that  forsake  the  rock 
for  the  ocean  a  few  days  before  or  after  the 
first  full  moon  in  October  or  November.  Their 
mode  of  migration  is  highly  characteristic.  Each 
worm  before  migration  consists  of  two  portions 
highly  contrasted  in  colour  and  structure.  The 
head-end  is  coloured  just  as  was  the  whole  body 
during  the  previous  year  and  is  provided  with 
the  normal  type  of  limb;  the  tail-end,  or  rather 
the  posterior  two-thirds  of  the  body,  is  now 
bright  green  or  blue  and  contrasts  markedly 
with  the  front  portion  in  colour,  in  its  strong, 
paddling  limbs  and  in  having  eyes  scattered 
along  its  sides.  On  the  appointed  evening,  all 
the  mature  worms  turn  round  in  their  burrows 
and  lie  with  their  tails,  instead  of,  as  usual,  with 
their  heads,  projecting  into  the  water.  Millions 
of  these  tails  now  proceed  to  revolve  and  in  a  few 
minutes  break  off  and  swim  away,  leaving  the 
severed  head-ends  in  the  burrow.  As  these 
detached  portions  separate,  their  activity  in- 
creases, and  guided  by  their  new  eyes,  strike 
away  from  the  shore,  spurting  out  eggs  and  milt 
as  they  go.  In  a  few  minutes  the  sea  is  con- 
verted to  the  consistency  of  vermicelli  soup,  as 
the  dying,  contracting,  convulsively  swimming 
disjecta  membra  cast  their  young  upon  the  ocean. 


182 


THE  ANIMAL  WORLD 


Fig.  19.  —  The  Palolo-worm  (Eunice  viridis)  from  Samoa. 
A  similar  Palolo  occurs  on  the  coasts  of  Florida  and  of 
Japan. 

This  worm  consists  of  a  thick,  annulated  head-end 
and  a  long  thinner  coiled  tail-end.  The  former  is  red- 
dish, the  latter  blue,  owing  to  the  contained  eggs  (the 
specimen  is  a  female).  The  long  tail-end  (bearing  eye- 
spots)  twists  itself  off  at  the  last  quarter  of  the  moon 
in  November  and  October  and  swims  out  to  sea.  The 
head-end  remains  in  the  rocky  burrow  and  regenerates 
a  new  tail.  (After  Wood  worth.) 


THE  CARE  OF  THE  YOUNG   183 

Meanwhile  the  natives  of  the  country  having 
feasted  in  honour  of  the  occasion  proceed  to  a 
great  haul,  and  the  fate  of  many  thousands  of 
Palolo  is  arrested  at  an  early  stage  of  develop- 
ment. The  severed  heads  of  the  Palolo  remain 
ensconced  in  their  burrows.  The  wound  soon 
heals  and  growth  of  a  new  tail  takes  place,  so 
that  by  the  time  the  next  October  moon  comes 
to  the  full  the  same  procedure  is  gone  through 
(Fig.  19). 

Even  in  our  own  temperate  seas  there  are 
annelids  which  repeat  this  remarkable  severance 
and  regeneration  of  tail-ends.  Under  the  boul- 
ders at  low-tide  there  may  often  be  found  in 
spring  a  yellow-banded  Myrianida,  trailing  a 
slender,  whitish  tail.  On  close  examination  the 
tail  breaks  up  into  a  number  of  joints,  but  these 
joints  are  each  an  independent  and  small  repro- 
duction of  the  parent,  with  the  important  differ- 
ence that  whilst  the  parent  is  sexless,  this  chain 
is  either  a  row  of  males  or  of  females.  These 
form  in  fact  the  segmented  mature  tail-end. 
They  separate,  swim  away  and,  after  discharging 
their  eggs,  become  transposed  into  sexless  speci- 
mens until  the  sext  spring,  when  they  reproduce 
the  same  phenomena;  the  head-end  meantime 
grows  a  new  tail,  which  for  a  year  exhibits  none 
of  these  startling  capacities  (Fig.  20). 

PROTECTION    OF    EGGS. — This    habit    of    dis- 


184 


THE    ANIMAL   WORLD 


charging  eggs  broadcast  on  the  water  is,  however, 
not  the  only  one.  Various  marine  animals  fix  a 
series  of  clutches  to  stones,  weeds,  and  even  carry 


Fig.  20. —  This  series  of  figures  illustrates  the  mode  of  division 
of  a  marine  worm,  Myrianida,  to  form  a  chain  of  young 
sexual  forms  (E) . 

The  body  divides  into  two  portions,  the  anterior 
portion  remaining  sexless,  the  posterior  portion  being 
pushed  further  and  further  back  by  a  new  zone  of  growth, 
which  produces  the  chain  of  sexual  individuals.  (  X  1.) 
(After  Malaquin.) 

them  on  their  own  persons.  Such  eggs  are  usu- 
ally protected  by  a  special  envelope:  either  a 
band  of  jelly  in  which  they  are  spaced  out  in 
cross  lines  and  spirals,  or  a  shell.  The  beautiful 


THE  CARE  OF  THE  YOUNG  185 

naked-gilled  molluscs,  Doris,  Eolis,  Aplysia, 
cover  the  coast  in  spring  time  with  their  egg- 
ribbons;  the  sea- worms  anchor  pear-shaped 
jellies  of  green  or  brown  eggs  in  the  sand;  the 
dogwhelk  fixes  its  flat  conical  eggs  in  rows  on  the 
eel-grass;  the  whelk  forms  a  large  cluster  of  im- 
bricated egg-scales  like  a  cone  (Fig.  23).  The 
gobies  and  sucker-fish  have  especially  careful 
methods  of  affixing  and  of  guarding  their  eggs, 
and  this  habit  forms  the  first  stage  in  the  forma- 
tion of  nests,  in  which  craft  fishes  are  more  adept 
than  is  commonly  understood. 

NEST-BUILDING  FISH. — The  goby  (of  which 
there  are  many  kinds)  selects  the  clean  valve  of 
a  clam  and  uses  this  as  the  ready-made  nest. 
The  pair  (for  the  goby  mates  with  but  one  and  is 
jealous  of  any  rival)  hover  round  an  inverted 
valve  and  then  the  male  scoops  out  the  sand 
from  underneath  it,  forming  a  cavity,  the  shell 
being  slightly  tilted  and  pressed  into  the  sand. 
The  female  then  enters  the  cavity  and  deposits 
her  eggs  on  the  lower  (inner)  surface  of  the  shell. 
These  eggs  are  somewhat  cigar-shaped  struc- 
tures, fixed  at  one  end  by  a  glutinous  network 
that  secures  them  firmly  to  the  shell.  Having 
done  her  work,  the  female  then  exchanges  places 
with  the  male,  who  remains  on  guard,  keeping 
up  a  constant  current  of  water  over  the  eggs  by 
movements  of  the  pectoral  fins,  and  darting  out 


186  THE  ANIMAL  WORLD 

at  the  approach  of  an  intruder.  Similar  "nests" 
are  made  by  the  sucker-family,  by  many  Pla- 
narians  and  Molluscs,  though  it  is  only  amongst 
fish  that  the  habit  of  guarding  the  eggs  is  adopted. 


B. 


A. 


Fig.  21.  —  A.  Doris,  the  sea-lemon  (half  natural  size).  This 
animal  belongs  to  the  naked-gilled  Mollusca  or  Nudi- 
branchs,  and  is  commonly  found  between  tide-marks. 
The  head  is  provided  with  a  pair  of  sensitive  tentacles 
(T),  and  the  gills  form  a  cluster  near  the  hinder  end  of 
the  body.  The  colouring  varies  greatly,  being  often 
of  a  lemon  yellow  with  purple  patches. 

B.  The  spawn  of  Doris  (X  1):  consisting  of  a  colourless 
band  of  eggs  arranged  in  a  spiral  and  fixed  to  the  under 
surface  of  a  stone.  The  number  of  eggs  in  such  a  cluster 
is  immense.  The  larva  which  escapes  from  these  eggs 
is  seen  on  Fig.  28. 

NEST  OF  STICKLEBACK. — True  nests  are  made 
by  many  different  kinds  of  fish.  The  common 
fifteen-spined  stickleback  associate  in  pairs,  and 
the  male  spins  a  kind  of  glue  with  which  he  binds 
fragments  of  sea-weed  together  into  a  tubular 
nest.  The  female  then  lays  her  eggs  and  these 


THE  CARE  OF  THE  YOUNG   187 

are  guarded  by  her  pugnacious  mate.     In  African 
rivers  many  fish  are  known  to  construct  large 


Fig.  22. —  Egg-capsules  of  Gasteropod  molluscs. 

A.  Capsules   of   Nassa   reticulata,    a    carnivorous   sea-snail. 

(  X    1.) 

B.  Mass  of  egg-capsules  of  the  whelk,  Buccinum.    (X  *.)  . 

C.  Spawn  of  a  Natica,  also  a  carnivorous  sea-snail.     This 

consists  of  eggs  bound  in  a  flat  spiral  by  grains  of  sand. 
(  X  1.)  (From  the  Cambridge  Natural  History.)  The 
larva  issuing  from  these  eggs  is  seen  on  Fig.  28. 

nests,  which  may  be  simply  tubular  or  built  of 
grass-stems  or  strengthened  with  layers  of  stones. 


188 


THE  ANIMAL  WORLD 


In  all  these  cases  of  nesting  habits,  the  young 
when  hatched  are  led  in  a  flock  by  the  male, 


Fig.  23.  —  Eggs  of  Gobius  minutus,  the  freckled  Goby,  at- 
tached to  a  shell  of  My  a,  the  clam.  At  B  is  seen  one  of 
the  eggs  highly  magnified,  showing  a  young  goby  almost 
ready  to  hatch.  The  large  brain  and  eyes  lie  near  the 
fixed  end  of  the  shell.  A  mass  of  yolk  is  as  yet  unab- 
sorbed.  The  markings  on  the  body  are  the  muscle- 
segments.  (After  Mclntosh  and  Masterman.) 


very  much  as  a  hen  leads  her  chickens.     Being 
cold-blooded,  however,  fish  do  not  incubate  their 


THE  CARE  OF  THE  YOUNG   189 

eggs,  and  therein  lies  one  of  the  chief  differences 
between  the  nests  of  birds  and  those  of  fish. 

REDUCTION  IN  SIZE  OF  FAMILY. — We  have 
spoken  above  of  the  vast  numbers  of  eggs  that 
are  produced  by  most  marine  animals  in  order  to 
compensate  for  the  destruction  of  the  greater 
part  of  their  young;  and  it  would  seem  natural, 
if  means  could  be  found  to  delay  hatching  until 
the  young  were  stronger,  that  such  species  should 
gain  a  certain  advantage.  This  step  would 
involve  the  disappearance  of  the  larval  stage  and 
the  retention  and  feeding  of  the  egg  for  a  longer 
time.  Comparatively  few  marine  animals,  how- 
ever, have  adopted  this  device  for  shortening  the 
childhood  of  their  race.  It  involves  great  re- 
duction in  the  number  of  eggs  and  enlargement 
of  their  size,  due  to  an  increased  amount  of  food- 
yolk.  No  doubt  if  marine  animals  were  able  to 
raise  the  temperature  of  their  body  above  that  of 
the  sea,  such  a  method  of  reducing  the  size  of  the 
family  and  increasing  the  size  of  each  member, 
would  be  advantageous.  Birds,  for  example, 
clearly  show  how  rapidly  by  this  means  they  can 
ensure  the  hatching  and  development  of  the  nes- 
tling through  the  critical  stages.  But  it  seems  as 
though  the  Malthusian  system  were  not  adapted 
for  most  coldblooded  creatures.  In  spite  of  this, 
there  are  some  remarkable  cases  of  its  adoption 
even  amongst  the  marine  animals. 


190 


THE  ANIMAL  WORLD 


LARGE  SIZE  OF  SHARK'S  EGGS. — All  sharks, 
dog-fish  and  skates  have  a  small  family  but  are 
born  of  a  relatively  large  size.  The  eggs  of  these 


Fig.  24.— -Nest  of  fresh-water  stickleback  (Gasterosteus 
pungitus)  constructed  of  weeds  matted  together  by 
the  male  fish.  The  eggs  are  seen  within.  (From  a 
specimen  in  the  Birmingham  Museum.)  (  X  1.) 

predaceous  fish  are  correspondingly  big.  A  dog- 
fish, two  feet  long,  has  eggs  that  measure  an  inch 
in  length,  or  with  the  egg-capsule,  two  inches. 
Such  "skate-purses"  are  well-known  objects  of 


THE  CARE  OF  THE  YOUNG 


191 


the  sea-shore.  Those  of  dog-fish  (Fig.  25)  are 
provided  with  fine  tendrils  which  entwine  round 
weeds  and  anchor  the  egg  at  such  a  depth  as  to 
allow  a  certain  essential  rocking  of  the  cradle  to 
be  made  by  the  waves.  Skate  lay  their  purses 
on  or  in  sand;  and  the  young,  at  hatching,  are 
already  miniatures  of  their  parent. 


Fig.  25. —  The  egg-case  of  the  dog-fish  moored  by  its  ten- 
drils to  seaweed.     (  X  |.) 

Some  sharks  that  live  in  deep  water  in  the 
Mediterranean  and  off  Japan,  lay  enormous  eggs, 
very  few  of  which  have  been  seen.  Perhaps  the 
quaintest  egg  laid  by  any  animal  is  that  of  the 
Chimcera,  or  of  its  fellow  in  Antarctic  water, 
Callorhynchus.  These  capsules  are  leaf-like  and 
provided  with  a  series  of  slits  through  which 
water  can  be  renewed  for  the  breathing  of  the 
contained  young. 

FAMILIES  OF  FRESH-WATER  ANIMALS. — This 
reduction  in  the  number  of  the  family  accompan- 
ied by  an  increase  in  the  size  of  the  infant  at 


192  THE  ANIMAL  WORLD 

hatching,  is  a  widely  accepted  principle  amongst 
fresh-water  animals  and  the  cause  of  its  adoption 
is  not  far  to  seek.  The  nestling  or  young  is  no 
longer,  when  hatched,  a  larva  with  habits  and 
structure  entirely  different  from  those  of  its 
parents.  On  the  contrary,  it  agrees  closely  in 
these  respects  with  its  parents,  and  is  therefore 
able,  as  a  rule,  to  take  up  its  residence  near  its 
place  of  birth,  unless  driven  out,  as  often  happens, 
by  the  "right  to  a  pitch"  which  obtains  with 
animals  as  with  man.  In  fresh  water  such  a 
direct  development  from  the  young  to  a  mature 
state  is  a  great  advantage,  owing  to  the  incon- 
stancy of  conditions  as  compared  with  the  com- 
paratively monotonous  sea.  Fresh  water  has 
rarely  the  depth  or  the  extent  requisite  for  main- 
taining even  some  degree  of  constancy.  Fresh 
water  runs  fast  or  slow,  it  evaporates,  becomes 
heated.  It  freezes  comparatively  quickly.  It 
is  pressed  back  by  tidal  waves  or  spreads  out  in 
flood  time  beyond  its  normal  limits.  Conse- 
quently for  fragile  larvae,  fresh  water  is  eminently 
unsuitable,  and  only  a  comparatively  small 
number  of  animals  are  known  to  go  through  a 
larval  history  in  fresh  water.  The  pond-mussel, 
the  zebra-mussel  (Dreissena),  the  Amphibia,  the 
lamprey,  certain  insects  and  the  Copepods  (or 
minute,  rowing  Crustacea  such  as  Cyclops)  are 
the  chief  of  these. 


THE  CARE  OF  THE  YOUNG    193 

SUPPRESSION  OF  MUCH  LARVAL  LIFE  IN  FRESH 
WATER. — This  fact,  the  incompatibility  of  free- 
swimming  larval  life  with  existence  in  fresh  water, 
explains  why  so  many  groups  of  marine  animals 
have  never  entered  fresh  water,  and  why  others 
have  only  a  meagre  representation  in  it.  No 
echinoderms  have  ever  lived  in  rivers  or  lakes. 
No  anemones,  corals,  or  large  Medusae  occur  in 
them.  The  Ascidians  and  cuttlefish,  king  crabs, 
certain  groups  of  worms,  and  great  numbers  of 
fish  are  always  absent. 

In  nearly  every  case  the  explanation  lies  in 
their  larval  history.  Animals  that  cannot  pro- 
duce eggs  that  develop  into  young  like  the  par- 
ents are  unable  to  obtain  a  footing  in  this  change- 
able medium:  and  even  some  of  those  (such  as 
cuttlefish)  which  produce  large  young  are  de- 
barred from  the  fresh-water  life,  probably  on 
account  of  the  almost  impassable  barrier  of 
brackish  water  that  lies  between  the  open  sea 
and  the  river. 

CARE  OF  THE  YOUNG  AMONGST  AMPHIBIA. — 
The  eggs  of  frogs  and  of  certain  insects  are  very 
easily  investigated.  Early  in  the  year  the  frogs 
wake  from  their  long  winter  sleep  in  the  pond 
mud  and  migrate  into  the  shallows,  where  they 
lay  their  spawn  without  artifice.  This  consists 
of  eggs  enclosed  in  an  albuminous  froth  compa- 
rable to  a  number  of  eggs  broken  into  a  basin. 


194  THE  ANIMAL  WORLD 

Each  egg  has  a  black  upper  pole  and  a  white 
nether  pole:  and  is  spaced  out  and  prevented 
from  crowding  upon  its  fellows  by  the  "white 
of  egg."  The  parents  take  no  care  of  the  eggs 
and  make  no  attempt  to  form  a  nest  for  their 
reception.  Toads,  however,  fix  their  eggs  in 
strings  to  water-weeds,  and  newts  glue  them  singly 
to  leaves.  In  exceptional  cases  newts  and  frogs 
attempt  to  glue  the  leaves  of  a  shoot  together  in 
order  to  form  a  rough  nest.  Among  tropical 
frogs,  however,  there  is  an  interesting  series  of 
nursing  habits.  The  male  of  Alytes,  the  obstetric 
frog  of  Europe,  carries  the  eggs  wound  about 
his  legs.  The  South  American  Rhinoderma 
carries  them  in  his  cheek-pouches.  The  Sur- 
inam toad  carries  the  eggs  on  her  back;  in  some 
species  the  eggs  are  large  and  the  young  escape 
in  the  perfect  state,  in  others  they  emerge  as 
tadpoles  and  go  through  their  development  in 
the  water. 

AQUATIC  INSECT-LARVAE. — Few  insects  except 
beetles  are  permanently  aquatic,  but  great  num- 
bers pass  the  earlier  stage  of  existence  in  water. 
Gnats,  mosquitoes,  dragon-flies,  may -flies,  stone- 
flies  and  many  others  lay  their  eggs  in  the  borders 
of  pools,  streams,  or  even  in  casual  water.  The 
mother,  however,  gives  little  care  to  her  young; 
the  eggs  are  either  dropped  singly  (dragon-flies) 
or  in  a  group  (drone-fly),  or  in  a  band  or  string 


THE  CARE  OF  THE  YOUNG   195 

of  jelly  (mosquito).  Aquatic  insects  do  not  at- 
tempt to  make  a  nest  (though  a  few  deposit  their 
eggs  in  the  roots  of  plants)  nor  do  they  guard 
their  eggs  or  young.  Though  we  call  some  of 
them  aquatic,  all  insects  are  essentially  aerial 
creatures,  and  even  the  water-beetles  fly  at  night. 
Fresh  water  is,  from  the  insect  point  of  view, 
merely  one  of  their  vast  nurseries,  useful  in  yield- 
ing an  abundant  supply  of  nutritious  food,  but 
useful  only  for  a  time:  and,  when  the  supply 
has  served  the  purpose  of  providing  enough  mate- 
rial for  the  full  development  of  these  dominant 
creatures,  the  water  is  forsaken  for  the  next 
stage,  on  which  the  last  act  of  insect  life  is  played 
out.  Hence,  perhaps,  the  perfunctory  way  in 
which  eggs  are  dropped  by  a  parent  Dragon-fly 
who  the  next  moment  is  flying  through  the  air. 

WELFARE  OF  LAND-FAMILIES. — On  land  the 
welfare  of  the  young  is  most  carefully  provided 
for.  The  difficulties  of  life  here  reach  their  max- 
imum, and  not  only  does  the  parent  in  most 
cases  protect  her  family,  but  adopts  means  for 
their  welfare  which  have  no  parallel  amongst  the 
animals  of  other  media.  The  number  of  the 
family  becomes  even  smaller  than  in  fresh  water, 
the  eggs  become  correspondingly  large,  or  are 
retained  and  fed  for  a  prolonged  period  before 
birth.  All  the  resources  of  obtaining  heat  and 
of  excluding  cold  are  employed,  and  the  parents 


196  THE  ANIMAL  WORLD 

do  not  abandon  their  young  even  after  they  are 
fledged.  The  adaptations  for  these  purposes 
already  seen  in  fresh-water  animals  are  carried  to 
a  still  further  degree. 

DANGERS  OF  TERRESTRIAL  LIFE. — The  reason 
for  these  precautions  becomes  evident  when  the 
difficulties  and  dangers  of  terrestrial  life  are  con- 
sidered. The  temperature  varies,  often  consider- 
ably and  quickly,  in  a  way  unknown  in  water,  and 
not  only  the  changeableness  but  the  extremes  of 
heat  and  of  cold  are  such  as  never  occur  in  other 
media.  The  weight  of  the  body  tells  at  every 
step,  and  renders  movement  a  difficult  and  com- 
plicated art  as  compared  with  the  drifting,  al- 
most passive,  motion  of  aquatic  life.  Food  no 
longer  streams  by,  but  has  to  be  sought  diligently. 
Toughness,  the  note  of  land  life,  is  characteristic 
of  all  its  manifestations,  and  a  young  land  animal 
may  seek  far  before  it  lights  upon  a  toothsome 
morsel.  Moreover,  life  is  now  lived  in  the  full 
glare  of  daylight,  in  contrast  to  the  subdued 
tones  and  deep  shadows  of  the  water  life,  and 
the  difficulty  of  avoiding  watchful  enemies  is 
even  greater  than  in  the  depths. 

EGGS  AND  NESTS  OF  LAND-ANIMALS. — The 
modes  of  protecting  their  young  against  these 
dangers  is  well  seen  if  we  compare  land  animals 
with  their  nearest  aquatic  allies.  The  earth- 
worms lay  their  eggs  in  cocoons  which  are  stored 


THE  CARE  OF  THE  YOUNG   197 

with  nourishing  albumen  and  only  one  egg  sur- 
vives, but  the  survivor  is  stronger  than  the  just- 
hatched  water-worm  and  as  advanced  as  a  sea- 
worm  some  months  old.  The  house-fly  utilizes 
hot-beds  and  manure-heaps,  where  the  heat  of 
fermentation  enables  the  young  to  undergo  their 
transformation  in  a  week,  whilst  the  mosquito, 
which  breeds  in  water,  takes  three  weeks.  The 
land  snail  and  the  slug  lay  batches  of  a  few  large 
eggs  under  stones,  unlike  their  allies  in  the  sea 
and  pond,  which  lay  discs  or  bands  of  jelly  each 
containing  many  small  eggs.  The  reptiles  choose 
hot-beds  of  decaying  vegetation  or  banks  of 
sand  exposed  to  hot  sun,  and  lay  their  eggs  there. 
Birds  heat  their  eggs  with  their  own  breasts  and, 
like  mammals,  feed  their  young  for  some  time 
after  birth.  Undoubtedly,  however,  insects  show 
more  clearly  than  any  other  class  the  pressure 
of  land  life  and  the  most  complete  adaptation  to 
surmount  its  difficulties.  We  may,  therefore, 
refer  in  some  detail  to  the  care  of  the  young 
amongst  insects,  more  particularly  to  the  order 
Hymenoptera,  in  which  the  welfare  of  the  race 
is  most  assiduously  considered. 

CARE  OF  THE  YOUNG  AMONG  BEES  AND  WASPS. 
The  most  familiar  of  these  insects  are  by  no  means 
the  most  primitive.  The  hive-bee,  the  common 
wasp,  and  the  meadow  or  wood  ant  are  members 
of  highly  organized  communities,  and  to  find  a 


198  THE  ANIMAL  WORLD 

suitable  starting-point  we  have  to  go  to  less  fa- 
miliar examples  of  the  order,  to  the  solitary  bees 
and  the  solitary  wasps.  These  are  common  and 
easily  found  in  all  parts  of  the  country,  yet, 
owing  to  their  silent  flight,  inconspicuous  size 
and  colouring,  are  little  known  and  readily  mis- 
taken for  two-winged  flies,  whilst  Hymenoptera 
possess  four  wings. 

BURROWING  BEES. — If  we  go  along  a  sunny 
lane  or  field  track  in  early  summer  and  notice 
the  footpath  that  is  worn  by  the  side  of  the  road, 
we  shall  soon  see  little  patches  of  freshly  thrown 
earth  at  the  side  of  small  holes,  and  whilst  watch- 
ing them  we  may  see  small  hairy  bees  fly  into  or 
out  of  them,  and  become  aware  of  a  colony  that 
has  sunk  its  shafts  in  the  hard  earth  and  is  busy 
collecting  pollen  for  the  young.  Careful  exam- 
ination of  sunny  banks  where  Composites  and 
gorse  or  broom  abound,  soon  discovers  that  these 
bees  are  smaller  than  those  of  the  hive  and  fly 
without  the  hum  of  a  bumble.  The  smallest 
of  them  is  only  as  large  as  a  house-fly  and  nests 
in  bramble  stalks.  In  the  same  situations  soli- 
tary wasps  are  found,  either  resting  in  the  in- 
florescences or  busily  quartering  the  herbage  for 
caterpillars. 

The  habits  of  these  solitary  Hymenoptera  are 
entirely  different  from  those  of  the  social  kinds. 
Through  the  winter  they  have  hidden  in  the 


THE  CARE  OF  THE  YOUNG   199 

ground,  but  on  the  first  warm  spring  day  a  few 
may  be  seen  on  the  wing.     Later  on  both  female 


Fig.  26. — Showing  the  burrow  of  a  solitary  bee  (Andrena) 
made  in  pathways  and  fields.  The  central  shaft  serves 
to  carry  away  water  and  to  act  as  an  exit  to  the  perfect 
insect.  The  egg  and  pollen-food  are  shown  in  the  lower 
right-hand  figure  and  stages  of  development  in  the 
others.  (From  Riverside  Natural  History.) 


£00  THE  ANIMAL  WORLD 

and  male  are  actively  drilling  holes  with  their 
powerful  jaws  and  scraping  the  burrows  with 
their  fore-legs  and  kicking  it  clear  with  their 
last  pair.  When  the  burrow  is  long  enough,  a 
lateral  pocket  is  made  and  the  locality  is  fixed 
in  mind  by  repeated  circuits.  The  bees  then  fly 
off  in  order  to  secure  a  supply  of  pollen.  Laden 
with  this  the  bee  returns,  descends,  lays  an  egg 
in  the  pollen  mass  and  then  proceeds  to  drill 
another  excavation,  leading  off  from  the  main 
shaft.  The  process  is  repeated  until  several 
"pockets,"  all  opening  into  a  central  shaft,  are 
stored  with  nourishment  for  the  larvae  that  will 
presently  emerge  from  the  eggs.  There  is  no 
attempt  to  form  a  comb  nor  to  collect  or  store 
honey.  After  one  set  of  pockets  is  completed 
and  stocked,  another  gallery  is  sunk  and  the  proc- 
ess repeated.  In  the  case  of  the  leaf-cutter 
bees,  the  burrow  is  lined  with  segments  of  rose 
leaves,  and  the  carder-bee  uses  down  for  the  same 
purpose,  the  object  being  probably  to  protect 
the  grub  against  its  most  insidious  enemy,  damp 
and  "rot." 

SOLITARY  WASPS. — In  somewhat  the  same 
way  solitary  wasps  will  make  or  utilize  a  crevice, 
such  as  a  key-hole,  and  store  it  with  caterpillars 
or  spiders.  They  sting  these  animals,  numbing 
them  but  not  killing  them  outright,  and  then, 
often  with  exhausting  labour,  haul  them  to  the 


THE  CARE  OF  THE  YOUNG    201 

burrow.  The  effect  of  the  sting  appears  to  have 
a  wonderfully  preservative  effect,  so  that  the 
wasp-grub  which  hatches  out  of  an  egg  laid  in 
the  centre  of  this  provender,  has  an  ample  supply 
of  fresh  food  material  during  its  period  of  growth. 
The  memoirs  of  Fabre  and  of  Mr.  and  Mrs. 
Peckham  give  a  most  fascinating  picture  of  these 
habits  of  the  solitary  bees  and  wasps. 

SOCIAL  BEES. — The  change  from  the  solitary 
to  the  social  Hymenoptera  appears  to  have  come 
about  by  the  young  members  of  the  family  asso- 
ciating for  a  time  with  their  parents  instead  of  at 
once  commencing  life  on  their  own  account. 
Such  a  family  group  has  been  met  with  amongst 
the  otherwise  solitary  bees  called  Halictus:  the 
result  of  their  interaction  being  the  formation  of  a 
rough  kind  of  comb.  The  family,  however,  do 
not  hold  together  very  long,  the  children  mating 
and  finding  homes  of  their  own. 

BUMBLE-BEES. — In  bumble-bees  the  stock- 
mother  makes  a  large  cellar  or  excavation  in  a 
bank  during  the  early  spring.  In  this  she  con- 
structs a  few  loosely  arranged  wax-cells,  and 
after  placing  an  egg  in  each  fills  them  up  with 
pollen  and  honey.  This  first  batch  develops 
into  a  brood  of  small  females  who  remain  and 
assist  in  the  extension  of  the  colony.  These  are 
the  "  workers "  though,  unlike  the  hive-bee 
workers,  many  of  them  are  capable  of  mating. 


202  THE    ANIMAL   WORLD 

Their  numbers  increase  as  batch  after  batch 
remain  in  attendance  on  the  mother-bee,  who 
now  rarely  stirs  out.  Finally,  towards  the  end 
of  the  season,  a  batch  of  male  bees  ensues,  and  a 
few  specially  large  cells  are  constructed  for  the 
growth  of  the  queens.  These,  without  swarming 
or  performing  a  nuptial  flight,  mate  with  the 
drones  and  hibernate  until  the  next  spring.  The 
workers  are  worn  out  with  the  labours  of  the  nest 
and  appear  to  succumb  before  the  end  of  the 
season. 

MOSQUITO-BEES. — The  Meliponas  or  mos- 
quito-bees of  the  East  seem  to  give  the  con- 
necting link  between  the  bumble-bees  and  the 
true  hive-bees,  though  little  is  as  yet  known 
about  them.  They  form  a  definite  comb  in 
hollow  trees,  the  worker-cells  being  hexagonal 
in  plan.  There  are  the  three  usual  castes,  queen, 
drone,  and  worker,  but  the  drones  are  not  yet 
idle,  luxurious  members  of  society. 

HIVE-BEE.  — Finally  the  hive-bee  exhibits  the 
well-known,  highly  organized  society  that  is  now 
only  met  with  under  cultivation.  In  this  insect, 
there  is  no  doubt  that  the  queen  decides  the  sex 
of  her  offspring,  but  the  conversion  of  a  female 
egg  into  a  queen  or  into  a  worker  is  determined  by 
the  nature  of  the  food  supplied  to  it  during  its 
early  development  by  the  workers. 

NESTS  AND  HABITS  OF  ANTS. — No  such  evo- 


THE  CARE  OF  THE  YOUNG    203 

lutionary  evidence  is  as  yet  forthcoming  in  the 
case  of  the  ant,  for  solitary  ants  are  unknown 
and  have  either  been  supplanted  by  the  more 
successful  societies  or  occupy  an  extremely  se- 
cluded position.  The  care  of  ants  for  their  young 
far  exceeds  that  of  any  other  animal.  Their  young 
are  not  kept  in  isolated  cells,  surrounded  by  food, 
they  are  fed  directly  by  the  workers,  carried  about, 
cleaned,  and  personally  conducted  through  the 
whole  of  their  early  career.  The  society  consists, 
not  as  in  the  hive-bee  of  a  single  queen,  winged 
workers  of  a  single  size  and  drones,  but  of  several 
queens  which  are  winged  only  for  a  few  hours 
during  the  year,  of  workers  of  two  or  three  sizes 
which  never  become  winged,  and  of  males.  The 
nest  is  an  irregular  cavity  in  the  earth  with  runs 
uniting  its  different  portions.  It  may  be  stocked 
with  fungi,  aphides  and  other  living  sources  of 
nutriment,  but  it  contains  no  honey-tubs  or 
pollen.  The  young  are  carried  to  the  surface  on 
warm  days  and  to  the  deeper  recesses  on  cold 
ones.  They  are  fed  by  pre-digested  nourishment 
and  in  many  colonies  even  the  soldiers  may  re- 
quire to  be  fed  all  life  long.  In  order  to  obtain 
assistance  in  this  large  nursing  home,  raids  are 
made  by  many  ants  on  weaker  colonies  and  alien 
workers  are  enslaved  by  their  forest-gangs.  In 
these  ways  we  see  that  the  young  of  social  hymen- 
optera  are  not  left  to  feed  themselves  as  are  the 


204  THE  ANIMAL  WORLD 

young  of  the  solitary  forms,  but  are  provided 
with  one  or  more  predigested  foods  by  the 
workers.  The  nature  of  these  foods  determines 
the  status  of  the  individual,  soldier,  worker,  or 
queen. 

SUMMARY. — Looking  back  over  this  brief  sur- 
vey of  nursing  habits  we  find  isolated  examples 
of  the  retention  of  the  young  in  many  marine 
animals,  accompanied  by  diminution  in  the  size 
of  the  family.  Speaking  generally,  marine  ani- 
mals pour  their  eggs  broadcast,  and  leave  their 
minute  larvae  to  complete  their  metamorphosis 
unaided  and  unsheltered,  but  in  every  group 
there  are  malthusian  species  which  not  only  re- 
strict their  families  in  number,  but  enclose  them 
by  protective  envelopes.  The  varied  experience 
of  larval  life  in  this  curtailed  and  direct  develop- 
ment supplants  metamorphosis.  The  eggs  become 
larger  and  the  young  stronger  at  birth. 

In  fresh  water  the  limitation  and  protection 
of  the  family  is  more  generally  the  rule.  Insects 
and  Amphibia  are  the  only  large  classes  which 
go  through  their  larval  life  in  fresh  water.  But 
the  protection  is  usually  of  the  simplest  kind  and 
is  confined  to  the  earlier  stages  of  development. 

On  land  the  insects  form  the  only  class  in 
which  the  majority  still  pursues  a  free  larval 
history.  Others  develop  directly,  either  grow- 
ing up  into  full  stature  without  guidance  or 


LIFE-HISTORIES   OF   ANIMALS     205 

protection,  or  carried  and  fed  by  their  parents 
for  some  time  both  before  and  after  hatching. 
The  size  of  the  family  (again  with  the  exception 
of  insects)  is  further  reduced  pari  passu  with  the 
increased  amount  of  such  nutrition. 


CHAPTER  IX 

LIFE-HISTORIES   OF   ANIMALS 

IN  a  previous  chapter  we  have  referred  to  the 
two  chief  types  of  life-histories  presented  by  ani- 
mals. One  is  a  history  of  strikingly  contrasted 
events,  the  other  of  a  gradual  development  in 
which  phase  after  phase  succeeds  one  another  in- 
sensibly. In  one  of  the  first  phases  of  life  is  a 
minute  "  larva  "  swimming  freely  or  a  worm-like 
grub  slowly  pacing  the  earth;  in  the  other  the 
young  is  at  birth  a  miniature  of  its  parent.  We 
must  now  refer  to  a  few  cases  in  more  detail. 

LIFE-HISTORY  OF  PROTOZOA. — Among  Pro- 
tozoa the  life-histories  are  often  astonishingly 
eventful;  the  complexity  being  due  to  the  alter- 
nation of  dividing  and  of  true  reproductive 
phases.  An  Amoeba  (Fig.  1),  for  example,  may 
go  for  long  without  more  colour  in  its  life  than  is 
produced  by  division  into  two,  but  under  certain 
conditions,  the  nature  of  which  is  quite  unknown, 


206  THE  ANIMAL  WORLD 

its  nucleus  (that  is,  the  central  governing  point  or 
structure)  becomes  multiple  and  the  Amoeba 
passes  into  a  spherical  shape  enclosing  itself 
in  a  thick  wall  or  cyst.  This  is  the  signal  for  re- 
production. Within  this  protecting  cyst  the 
nuclei  break  up  into  fragments  and  organize 
minute  flagellated  bodies  around  themselves,  so 
that  the  Amoeba  is  converted  into  a  multitude  of 
active  bodies  which  appear  to  be  all  alike.  These 
bodies  (gamates)  mate  in  pairs,  each  pair  forming 
a  small  Amoeba  (zygote).  These  escape  from 
the  cyst  and  grow  to  their  full  size.  Thus  in  the 
very  simplest  of  animals  there  is  division  of  the 
life-history  into  three  events:  the  solitary  form, 
its  fission,  and  the  formation  of  gametes  which 
fuse  to  form  a  zygote.  These  events  form  two 
circles:  the  dividing  Amoebae  performing  the 
short  circuit,  the  multiplying  Amoebae  the  long 
one.  Such  a  history  is  typical  of  all  Protozoa, 
though  the  details  vary  in  different  cases.  The 
chief  variation  is  due  to  the  incomplete  separation 
of  the  fission  products  so  that  colonies  may  be 
formed.  Such  colonies,  however,  consist  of  iso- 
lated individuals  even  though  they  may  live  in  a 
common  jelly  or  on  a  common  stalk. 

DIVISION  OF  EGG-CELL. — In  higher  animals 
the  history  also  begins  as  a  single  cell.  This  cell 
is  a  double  structure  formed  by  the  union  of  two 
gametes.  It  is,  in  fact,  a  zygote.  Usually,  as  we 


LIFE-HISTORIES  OF  ANIMALS     207 

have  seen,  it  is  discharged  in  vast  numbers  upon 
the  surrounding  water,  but  it  may  be  retained 
for  a  varying  period  within  the  shelter  of  its 
mother.  This  cell  divides  (Fig.  28)  like  an 
Amoeba,  but  the  halves  do  not  separate,  since 
they  are  held  together  by  an  enclosing  membrane 
and  by  an  intercellular  cement.  Divisions  now 
follow  successively  until  a  ball  of  cells  is  formed, 
the  arrangement  being  determined  largely  by  the 
size  of  the  initial  cell,  and  this  again  is  due  to  the 
amount  of  food  material.  As  a  rule,  all  the  early 
cells  are  of  the  same  shape  and  size,  and  corre- 
spond to  a  number  of  Amoebse  held  together.  Be- 
fore long,  however,  the  divisions  become  asym- 
metrical and  the  cells  become  dissimilar.  Varia- 
tion steps  in  and  with  it  division  of  labour.  The 
ball  of  cells  becomes  a  sac  with  inner  and  outer 
linings  comparable  with  the  layers  of  a  Hydra. 
It  acquires  an  opening,  frequently  it  begins  to 
rotate  within  its  capsule,  and  long  before  it  ex- 
hausts its  store  of  nourishment,  starts  out  upon 
a  voyage  or  journey  unless  supplied  with  more 
food  by  its  mother. 

LlFE-HlSTORY    OF    SPONGES    AND    HYDROIDS. 

Amongst  sponges  the  eggs  are  usually  very  small 
and  therefore  give  rise  to  free-swimming  larvae. 
But  amongst  fresh-water  sponges,  in  accordance 
with  the  rule  that  larval  life  is  discontinued  when 
unwise  animals  colonize  fresh  water  the  eggs  are 


208  THE  ANIMAL  WORLD 

large,  and,  after  surviving  drought  or  resting 
through  the  winter,  develop  straightway  into  a 
small  fixed  sponge.  Amongst  hydroids  the  his- 
tory is  more  complicated.  The  ovum,  being 
minute,  develops  into  a  free-swimming  egg-shaped 
larva  which  drifts  helplessly  on  the  sea.  If  for- 
tunate in  finding  a  holdfast,  it  anchors  and  grows 
up  into  a  branching  colony  of  fixed  plant-like  as- 
pect. Along  the  course  of  these  branches,  however, 
certain  buds  of  a  peculiar  character  are  formed. 
Unlike  the  majority,  they  are  mouthless  and  en- 
closed in  a  protective  sheath.  These  buds  corre- 
spond to  the  cyst  of  Amoeba  and  within  them  pro- 
ceed the  changes  by  which  the  gametes  or  mating 
cells  are  produced  or  ripened.  In  some  hydroids 
they  remain  mere  sacs,  in  others  they  develop 
canals  and  eye  spots;  and  in  others,  again,  they 
show  their  proper  nature  by  becoming  jelly-fish, 
pulsatile  bells  with  clapper  hanging  in  the  centre 
(Fig.  29).  In  a  single  night,  dozens  of  such  medusae 
swim  away  into  the  sea  carrying  with  them  the 
mating  cells  and,  after  growing,  discharge  the  cells 
broadcast.  The  cells  unite,  form  a  zygote,which 
again  describes  the  complex  circle  of  existence  if 
spared  to  do  so.  In  such  a  history  the  phases  are 
— zy  gote — larva . 

yhydroid  buds 

hydroid^—medusoid  buds — medusa-zygote 
gametes. 


LIFE-HISTORIES  OF  ANIMALS     209 

There  is  thus  an  alternation  of  hydroid  with 
medusa-stages  or  generations.  As  in  the  case  of 
fresh-water  sponges  we  have  in  many  fresh-water 
hydroids  a  loss  of  the  active  phase  of  life,  for  in 
Hydra  it  is  not  only  the  special  medusa-buds  that 
are  suppressed  but  the  zygote  develops  straight- 
way into  a  young  Hydra. 


n  TT  j     ^x' 

Zygote  —  Hydra<^ 

gametes  —  zygote. 


Fig.  27.  —  Showing  the  division  of  an  animal  egg-cell  (Amphi- 
oxus,  a  primitive  gill-breathing  vertebrate).  The  single 
cell,  or  rather  zygote  (A),  is  dividing  into  two  at  B,  and 
these  again  into  two  each  at  C.  Each  of  these  four 
cells  is  equipollent,  that  is,  each  if  shaken  apart  from 
the  rest  is  capable  of  giving  rise  to  an  Amphioxiis,  but 
only  one  quarter  of  the  normal  size.  So  far  we  have 
a  simple  case  of  heredity. 

The  division  of  these  four  cells  at  D  into  eight  is  a 
case  of  variation.  The  four  upper  cells  have  a  different 
destiny  from  that  of  the  four  lower  ones,  and  none  of 
the  eight  is  capable  of  giving  rise  to  a  whole  organism. 

LIFE-HISTORY  OF  EcHiNODERMS. — In  Echino- 
derms,  Annelids,  and  Molluscs  we  see  the   two 


210  THE  ANIMAL  WORLD 

types  of  life-history,  according  to  the  size  of  the 
egg  and  the  habitat  of  the  parent.  Many  star- 
fish, sea-urchins,  and  brittle-stars  develop  from 
the  egg  into  a  larva  utterly  unlike  the  parent. 

In  place  of  the  firm  radiate  body  sluggishly 
moving  on  the  solid  ground,  the  larva  has  a 
transparent  two-sided  body  drawn  out  into  tenta- 
cles and  fringed  with  cilia  by  which  it  glides  easily 
through  the  water,  and  drinks  as  it  swims.  For 
weeks  to  come,  its  life  is  amongst  the  sunlit  layers 
of  the  ocean  where  it  jostles  with  the  hosts  of 
plankton.  Such  larvse  lead  a  double  life.  They 
are  almost  dual  animals,  for  there  grows  out  of 
their  left  side  a  "coelom"  (p.  35;  see  Fig.  36,  p. 
250)  which  is  almost  as  foreign  to  the  rest  of  the 
larva  as  a  parasite.  This  sac  has  within  its  sphere 
of  influence  a  portion,  and  a  portion  only  of  the 
larva.  Around  it  the  tissues  are  moulded  into  the 
form  of  the  future  star  of  echinus,  whilst  beyond 
that  modifying  influence  the  larva  still  pursues 
its  own  devices.  Presently  the  star  within  it 
acquires  a  mouth,  a  nervous  system,  and  loco- 
motor  organs,  whilst  the  larva  on  which  it  hangs 
has  still  its  own  mouth,  its  own  nervous  system, 
and  its  own  ciliated  bands.  This  organized 
growth,  however,  soon  exhausts  the  larva  that 
bore  it.  "My  need  is  greater  than  thine,"  is  its 
motto,  and  the  larva  is  presently  depleted  of  all 
its  material  in  order  to  feed  the  growth  that  is, 


LIFE-HISTORIES  OF  ANIMALS 

as  it  were,  imposed  upon  itself.  The  birth  of  Eve 
is  no  stranger  a  story  than  is  the  development  of 
a  starfish  or  sea-urchin  out  of  the  left  side  of  a 
larva. 

Loss  OF  LARVAL  LIFE. — In  contrast  to  this 
unequal  struggle  between  the  two  natures  in  pe- 
lagic, larval  echinoderms,  the  development  of 
protected  young  proceeds  unexcitingly.  Such 
echinoderms  live  in  small  families  and  are  nursed 
on  their  mother's  back.  They  are  often  en- 
closed in  a  special  nursery  fenced  in  by  spines. 
They  are  opaque,  fat  with  much  yolk,  and  in- 
capable of  rapid  movement.  But  even  these 
have  to  sacrifice  some  part  of  their  larval  organs, 
which  are  not  so  large  nor  so  highly  organized. 

LARV.E  OF  WORMS  AND  MOLLUSCS. — The  same 
story  applies  to  Annelids  and  Molluscs.  The 
minute  young  (Fig.  28)  are  cast  upon  the  ocean 
in  myriads.  They  develop  into  top-shape 
organisms  moved  by  cilia,  fed  by  cilia,  guarded 
by  a  primitive  nervous  system,  and  responding 
to  the  varied  impulses  due  to  light,  gravity, 
salinity,  and  other  agencies  of  the  open  ocean. 
But  in  addition  to  these  larval  organs  and  larval 
activities  there  are  the  rudiments  of  the  future 
Annelid  or  Mollusc,  again  in  the  form  of  out- 
growths from  the  body  of  the  larva.  We  have 
here,  therefore,  as  in  the  larval  history  of  starfish, 
a  dual  organism  with  two  sets  of  organs,  two  kinds 


THE  ANIMAL  WORLD 

of  tendencies.     One,  the  larva  proper,  is  simple: 
its  structure  is  on  a  level  with  that  which  wheel- 


Fig.  28.  —  A.  Free-swimming  larva  of  Doris,  a  marine  naked- 
gilled  mollusc.  The  body  is  enclosed  in  a  transparent 
nautiloid  shell,  from  which  a  pair  of  swimming  lobes 
project  provided  with  large  cilia  (highly  magnified). 

B.  Free-swimming  larva  of  a  Gasteropod,  showing  a  four- 
lobed  swimming  organ,  foot  and  coiled  shell.  A  pair 
of  eyes  are  present  at  the  base  of  the  two  tentacles 
(highly  magnified). 

animalcules  retain  throughout  life,  that  is,  an 
acoelomate  double  bag  with  ciliated  bands  for 


LIFE-HISTORIES  OF  ANIMALS    213 

locomotion  and  for  nutrition  and  a  simple  ner- 
vous plate,  the  whole  being  unsegmented. 

Posterior  to  this  larva,  and  growing  out  of 
it,  is  a  double  band  of  cells — the  so-called  germ- 
bands — which  has  quite  other  properties.  For 
example,  this  structure  is  coelomic,  it  grows 
rapidly  in  length,  it  is  segmented.  The  portions 
of  the  larval  tissues  adjacent  to  it  are  modified 
profoundly.  A  new  nervous  system  forms  from 
them,  the  segments  of  which  correspond  to  the 
divisions  of  the  germ  buds,  whilst  from  these 
coelomic  plates  or  bands  there  are  formed  the 
muscles,  kidneys  and  reproductive  organ  of  the 
future  worm.  The  dual  organism  now  looks 
like  a  stalked  bladder,  the  larva  or  head  part  being 
swollen  and  carrying  behind  it  the  segmented 
worm  as  a  sort  of  appendage.  Presently  the 
larval  tissues  are  sacrificed,  and  out  of  them  a 
head  is  fashioned  to  fit  the  body.  Here  again 
we  see  an  example  of  that  dominance  of  the 
coelom  so  obvious  in  the  Echinoderm  larva 
and  referred  to  in  our  first  chapter. 

LIFE-HISTORY  OF  EARTH-WORMS. — In  contrast 
to  this  sacrifice  of  larval  tissue  in  an  unequal 
contest  with  an  alien  and  higher  tissue,  there  is 
in  the  history  of  fresh-water  or  earth-worms  a 
smooth  course  of  development.  The  parent, 
who  is  both  mother  of  her  own  and  father  of 
another's  family,  forms  a  case  by  secreting  from 


214  THE  ANIMAL  WORLD 

her  girdle  an  elastic  ring.  Drawing  herself  through 
this,  she  slips  therein,  as  it  contracts,  the  egg 
and  a  store  of  yolk.  When  her  head  is  with- 
drawn the  whole  structure  closes  up  and  forms 
a  yellow  or  brown  pea-like  cocoon  in  which  the 
eggs  undergo  their  whole  development,  becom- 
ing carved  into  miniatures  of  the  parent  with 
hardly  the  loss  of  a  single  cell. 

STRUGGLE  FOR  EXISTENCE  AMONG  EMBRYOS. 
— But  though  spared  the  acute,  unequal  struggle 
between  the  needs  of  larval  and  of  adult  structure, 
these  earth-worms  have  difficulties  of  their  own 
as  great  as  those  of  their  marine  allies.  Being 
unable  to  adopt  a  free  life  until  hatched  in  the 
perfect  state,  it  is  essential  for  each  egg  to  pos- 
sess or  acquire  enough  yolk  for  the  purpose. 
Three  or  four,  perhaps  more,  eggs  may  com- 
mence development,  and  it  happens  that  the 
yolk  available,  though  enough  for  one  or  two, 
is  not  sufficient  for  all.  The  stronger  infants  are 
then  under  the  painful  necessity  of  devouring 
their  brothers,  and  this  apparently  they  do;  so 
that  out  of  a  promising  family  of  six  only  one 
survives.  The  same  strange,  internecine  strug- 
gle is  seen  among  molluscs  such  as  whelks,  which 
lay  large  eggs.  Each  capsule  contains  several 
young,  and  the  strongest  hatches  with  its  brothers 
inside  it. 

LIFE-HISTORIES  OF  INSECTS. — Of  all  life-his- 


LIFE-HISTORIES  OF  ANIMALS     215 

tories,  perhaps  those  of  insects  are  the  most 
attractive.  At  every  point  they  form  a  complete 
contrast  to  those  of  other  animals.  The  episodes 
are  so  shaply  defined,  the  change  of  habit  so 
marked,  and  the  whole  history  so  comparatively 
easy  to  follow  that  the  metamorphoses  of  insects 
have  received  more  attention  than  any  others. 

These  histories  are  of  three  kinds.  First, 
there  is  the  most  usual  episodic  type,  with  a 
larval  preface,  a  chrysalid  text,  and  an  imago 
conclusion.  This  kind  occurs  in  most  orders. 
Secondly,  there  is  the  history  with  less  moving 
incident  by  flood  and  field.  In  these  cases  the 
young  generally  resemble  the  adults,  but  are 
without  wings.  Lastly  the  life-history  may  be 
completed  without  any  marked  change  of  ap- 
pearance. We  may  take  examples  of  these 
histories  in  the  inverse  order. 

DIRECT  DEVELOPMENT. — The  simplest  insects 
live  amongst  moss,  under  stones  on  the  shore, 
amongst  grass,  and  even  in  kitchens.  They 
occur  in  all  latitudes,  and  very  similar  kinds 
live  on  our  coasts  and  those  of  the  Antarctic. 
They  hatch  with  the  full  number  of  segments, 
and  never  possess  wings.  This  direct  mode  of 
development  is  probably  the  ancestral  one,  and 
the  complicated  histories  we  are  about  to  de- 
scribe appear  to  be  adaptations  to  secure  either  a 
more  rapid  development  or  a  more  complex  life. 


216  THE  ANIMAL  WORLD 

GRADUAL  METAMORPHOSIS. — The  next  stage  is 
furnished  by  the  Hemiptera  (scale  insects,  aphides, 


Fig.  29.  —  Sarsia:  a  medusa  budded  off  from  a  hydroid. 
This  hydroid  is  a  creature  like  Hydra,  but  branched 
and  producing  two  kinds  of  buds:  hydroid  or  feeding 
buds,  which  remain  attached  to  the  parent,  and  medu- 
soid  buds,  which  feed  and  breed,  break  away  from  the 
parent-stock  and  acquire  a  new  type  of  organization. 

The  medusa  consists  of  a  bell,  from  the  centre  of 
which  a  long,  hollow  clapper  hangs  down,  ending  in  the 
mouth.  This  long  tongue  has  the  power  of  budding 
fresh  medusae  as  shown. 

Sarsia,  like  nearly  all  medusae,  is  purely  marine,  trans- 
parent and  colourless.  (  X  8.) 

etc.)  and  Orthoptera  (locusts,  grasshoppers,  and 
cockroaches),  in  which  the  young  are  also  born 


LIFE-HISTORIES  OF  ANIMALS     217 

with  the  full  number  of  segments,  and  feed  on 
the  same  diet  as  do  their  parents,  but  differ 
from  these  in  not  being  provided  with  wings. 
Generally  (see  Fig.  30)  the  wings  are  only  gradu- 
ally acquired  as  the  animals  become  mature, 
the  attainment  of  their  full  spread  and  size  being 
a  sign  of  that  maturity;  but  in  many  plant-lice 
eggs  may  be  laid  by  specimens  that  have  no  wings 
and  the  life-history  is  complicated  by  the  mi- 
gration of  these  destructive  insects  from  one 
food-plant  to  another. 

LIFE-HISTORY  OF  APHIS. — If  we  take  the 
common  rose  aphis  as  an  example,  we  find  during 
the  summer  vast  numbers  of  red  or  green  speci- 
mens on  the  terminal  shoots  of  their  host-plant. 
Some  of  these  are  winged,  some  are  wingless. 
On  isolating  either  kind  on  a  shoot  placed  in 
water  it  will  be  found,  in  the  course  of  a  day  or 
two,  accompanied  by  a  little  family  of  young 
of  the  same  colour  as  the  parent.  On  again 
isolating  one  of  these  and  feeding  it  with  a  juicy 
shoot  in  a  warm  situation,  it  will  be  found  in 
the  course  of  a  fortnight  to  give  rise  to  another 
family,  and  so  on.  This  rapid  succession  of 
broods  (which  may  be  winged,  wingless,  or 
mixed)  is  composed  entirely  of  one  sex:  self- 
sufficient  females;  and  in  captivity  this  history 
may  be  indefinitely  continued  without  the 
formation  of  eggs  or  the  production  of  males, 


218  THE  ANIMAL  WORLD 

nor  is  it  certain  that,  even  under  natural  con- 
ditions, egg-laying  female  rose-aphis  appear 
except  at  extremely  rare  intervals. 

In  other  species,  however — the  hop-aphis, 
for  example — the  history  is  more  complicated, 
for  the  species  spends  part  of  the  summer  on  the 
hop  and  part  on  the  plum,  apparently  needing  to 
oscillate  between  the  two  food-plants  in  order 
to  keep  up  a  supply  of  young.  All  the  specimens 
produced  during  the  summer  are  self-sufficing 
females,  and  all  the  broods  are  born  as  miniatures 
of  the  parent,  and  not  laid  as  eggs.  At  the  ap- 
proach of  winter,  however,  and  under  conditions 
that  are  far  from  being  understood,  the  young 
develop  into  two  new  forms  differing  very  little 
externally  from  the  preceding  generations,  with 
the  capacity  to  grow  up  into  males  (winged 
or  wingless)  and  females  (winged  or  wingless). 
In  some  species  these  mutually  necessary  and 
complementary  forms  occur  at  regular  intervals 
of  so  many  viviparous  generations,  but  in  others 
they  appear  at  irregular  intervals,  generally  at 
the  onset  of  cold  weather  and  coincidently  with 
the  hardening  of  the  tissues  of  the  host-plant. 
Such  females,  unlike  those  of  the  summer  gener- 
ations, lay  eggs  which  remain  dormant  through- 
out the  winter  and  only  hatch  at  the  coming  of 
spring.  Each  egg  then  gives  rise  to  a  "stock- 
mother"  from  which  the  summer  host  is  derived. 


LIFE-HISTORIES  OF  ANIMALS       219 

This  example  of  the  life-history  of  plant-lice 
shows  that  the  complicated  cycle  is  not  con- 
cerned with  the  perfecting  of  a  complex  organi- 
zation (as  in  the  higher  insects)  so  much  as  with 
the  rapid  production  of  colonies  during  the  most 
favourable  time  of  the  year. 

COMPLEX  METAMORPHOSIS. — In  the  other  in- 
sect orders  the  development  almost  invariably 
begins  with  a  hard-shelled  egg.  The  parents 
being  almost  without  exception  aerial  creatures 
usually  provided  with  wings  and  feeding  upon 
fluid  substances,  the  life-history  is  divided  into 
two  distinct  parts:  a  period  of  rapid  growth 
under  conditions  of  abundant  nourishment  and 
little  exertion,  and  another  of  complicated  de- 
velopment during  which  the  organs  for  aerial 
life  are  perfected  and  then  employed.  The 
period  of  growth,  or  larval  period,  is  largely  one 
of  mere  increase  of  bulk,  but  as  the  life  of  the 
"fly"  is  so  utterly  different  from  that  of  the 
larva  and  requires  organs,  adjustments,  and 
sensations  of  an  entirely  different  order,  it  is  rarely 
possible  for  the  larval  life  to  lead  straight  up 
to  that  of  the  adult.  Hence  comes  the  necessity 
of  a  transition  period  when  the  organs  of  the 
larva  may  be  transformed  or  abolished  and 
those  of  the  perfect  insect  developed.  This 
transition  state  is  the  pupal  or  chrysalis  period. 
According  as  the  changes  during  this  period  are 


220 


THE  ANIMAL   WORLD 


gradual  or  violent,  so  the  pupa  is  active  or  motion- 
less. 

The  most  familiar  case  of  this  complex  life- 
history  is  that  of  butterflies,  but  the  incidents 
are  much  the  same  in  moths,  beetles,  dragon- 
flies,  two-winged  flies  and  Hymenoptera.  The 
perfect  insects  are  active,  winged  creatures  adap- 


Fig.  30.  —  Illustrating  the  gradual  development  of  wings  in 
the  grasshopper.    (After  Packard.) 


ted  for  living  either  on  the  nectar  of  flowers  or 
the  juices  and  flesh  of  prey.  Their  larval  state 
is  worm-like,  immobile,  or  at  least  inactive, 
and  sustained  by  a  bacterial  of  vegetarian  diet — 
a  flesh  diet.  The  greatest  accumulation  of 
fresh  or  decaying  vegetation  is  found  on  land 
round  the  borders  of  sea  and  fresh  water.  Hence 
the  eggs  of  such  insects  are  laid  in  accordance 
with  the  relative  abundance  of  fresh  shoots  on 
the  leaves  of  plants  or  near  the  margin  of 


LIFE-HISTORIES  OF  ANIMALS 

water.  There  is,  however,  hardly  a  situation 
in  which  insects  are  not  found.  They  inhabit 
the  most  poisonous  drug  or  the  most  populous 
cities,  as  well  as  the  open  ocean  and  the  highest 
mountains,  and  their  larvae  have  an  almost 
equal  facility  of  adapting  themselves  to  con- 
ditions of  almost  every  kind.  The  majority, 
as  we  have  said,  feed  on  waste  nitrogenous  sub- 
stances, or  on  the  verdure  of  the  earth,  but  there 
are  many  which  attack  timber,  cloth,  books,  and 
dried  substances  of  any  kind. 

LIFE-HISTORY  OF  DiPTERA. — Perhaps  the 
most  remarkable  instances  of  larval  adaptation 
to  varied  surroundings  are  found  amongst  the 
Diptera,*or  two-winged  flies  (including  the  various 
forms  of  flies  found  in  houses,  the  blood-sucking 
flies,  the  daddy-long-legs  or  Tipulids,  the  midges 
and  tsetse-fly).  The  common  harlequin  fly, 
or  midge,  Chironomus  for  example  of  which 
there  are  two  hundred  British  species,  lays  its 
eggs  in  casual  water,  on  the  sea-shore,  in  running 
streams  (Fig.  31);  whilst  closely-allied  biting 
midges  (Ceratopogori)  breed  under  bark  and  in 
wet  hollows  amongst  timber  (Fig.  33).  The 
buffalo-fly  (Simuliwri),  which  is  such  a  plague 
to  man  and  beast  in  most  parts  of  the  world, 
though  harmless  in  Britain,  lays  its  eggs  in  rapid 
streams;  whilst  the  gaily-coloured  hover-flies, 
according  to  their  kind,  seek  out  rose-bushes 


222 


THE  ANIMAL  WORLD 


covered  with  green-fly,  a  putrescent  pool,  or  the 
nest  of  some  bee  or  wasp. 

The  larvse  that  hatch  from  eggs  placed  in  such 
various    situations    offer    a   bewildering    variety 


Fig.  31. — Life-history  of  Chironomus  pusio,  a  gnat  of  moun- 
tain and  moor. 

A.  The  egg-mass  fastened  to  a  leaf  in  a  Devonshire  stream. 

B.  The  cases  made  by  the  larvae  and  pupse  whereby  they 

anchor  themselves  and  catch  prey  (a  pupal  case  is  seen 
on  the  extreme  left) . 

C.  A  larva  magnified,  showing  the  antennae,  jaws  and  tho- 

racic feet.    At  the  end  of  the  body  are  two  abdominal 
hooked  feet. 

D.  The  pupa  in  its  case.    (After  Mundy.) 


of  external  form.  Some  (such  as  those  of  gnats) 
are  active,  red,  green  or  colourless  worm-like 
creatures,  sometimes  inhabiting  tubes,  sometimes 
pursuing  a  roving  life.  The  tubicolous  species 
possess  divers  sorts  of  casting  nets  or  collecting 


LIFE-HISTORY  OF  ANIMALS      223 

hairs  by  which  they  intercept  the  drift-life  of 
the  streams  and  inhale  it,  others  (such  as  the 
rat-tailed  larva  of  the  drone-fly)  simply  engulf 
the  decaying  matter  of  some  standing  pool.  The 
leather- jacket  or  larva  of  crane-fly  eats  the  roots 
of  herbage;  the  frit-fly  larva  devours  turnips; 
that  of  the  Hessian  fly  injures  the  stems  of  corn. 
So  great  is  this  adaptive  modification  of  struc- 
ture to  habit  that  quite  similar  flies  may  have 
very  different  larvae.  The  general  principle  of 
structure,  however,  is  much  the  same  in  all.  The 
head  of  the  larva  is  usually  small;  the  body  is 
uniformly  segmented,  and  not  divided  into 
thorax  and  abdomen  as  is  that  of  the  fly;  and 
the  organization  is  such  as  to  enable  rapid  growth 
to  take  place  without  depletion  of  the  stores  of 
energy  and  material  needed  for  active  sustained 
movement  or  for  the  development  of  complex 
sensory  organs. 

The  length  of  larval  life  amongst  the  Diptera 
varies  greatly,  partly  according  to  the  nutritious 
character  of  the  food,  partly  according  to  the 
temperature  of  the  surroundings.  A  house-fly 
may,  under  warm  conditions,  complete  its  larval 
development  in  four  days;  under  less  favourable 
surroundings  it  may  last  from  two  to  three  weeks. 
In  the  same  way  the  larval  life  of  other  Diptera 
varies  according  to  the  temperature  and  the 
stimulating  or  impoverished  nature  of  their 


THE  ANIMAL  WORLD 

food.  In  a  few  remarkable  flies,  of  which  the 
dreaded  tsetse-fly  (Glossina)  is  one,  the  larval 
stage  is  passed  in  the  body  of  the  mother,  and 
the  fly  is  born  as  a  full-grown  larva  which  im- 
mediately changes  into  the  pupal  state. 


Fig.  32. —  Ascidians,  or  sea-squirts. 

A.  A  simple  solitary  form,  showing  the  ingoing  and  outgoing 

currents  of  water  that  provide  it  with  food  and  oxygen. 
(X  i.) 

B.  A  colonial  form  (Botryllus),  consisting  of  six  individuals, 

each  with  a  separate  inhalent  opening  but  only  sharing 
in  a  common  exhalent  pore.  (  X  6.) 

C.  Shows    the    free-swimming    larva    of    this   group.      The 

larva  is  tadpole-shaped,  and  the  tail  is  provided  with 
an  upper  arid  under  fin.  The  study  of  this  larva  has 
led  to  the  discovery  of  the  true  position  of  Ascidians 
in  the  animal  kingdom.  (  X  60.) 

THE  CHRYSALIS-STAGE. — This  pupal  or 
chrysalid  state  is  requisite,  as  we  have  said, 
owing  to  the  reconstruction  of  the  larval  tissues 
to  form  those  of  the  fly.  In  some  gnats  the  pupa 
is  never  quite  inactive,  but  in  most  flies  the 
changes  that  go  on  within  it  are  so  great  as  to 


t  LIFE-HISTORIES  OF  ANIMALS     225 

render  a  motionless  period  absolutely  necessary. 
During  that  period  the  muscles,  skin,  alimentary 
canal,  and  other  organs  (except  the  nervous 
system  and  the  vascular  system)  are  eaten  up; 
then  from  the  pulp  there  is  produced  the  material 
for  wings,  legs,  antennae,  air-sacs,  and  other 
organs.  The  nervous  mechanism  is  perfected, 
and  when  hatched  and  dry  the  fly  makes  its  first 
circuit  as  accurately  as  if  it  had  practised  the 
movement  for  days. 

DEVELOPMENT  OF  VERTEBRATES. — The  life- 
histories  of  higher  animals  can  only  be  very 
lightly  dealt  with  in  such  a  sketch  as  this.  The 
fixed  sea-squirts  illustrate  the  larval  type  of 
history.  These  strange  drinkers  and  sifters 
of  the  sea  cover  the  rocks  and  stones  of  our 
coasts,  and  were  for  long  placed  with  the  In- 
vertebrates on  account  of  their  unsegmented 
bodies,  unmarked  by  any  complexity  of  struc- 
ture except  the  filtering  throat.  Their  develop- 
ment completely  altered  such  conception  of 
their  place  in  nature,  for  it  was  found  that 
they  hatch  as  Tadpole-like  organisms  provided 
with  a  tubular  nervous  system  underlain  succes- 
sively by  the  forerunner  of  the  vertebral  column 
and  by  the  pharynx.  This  combination  of 
characters  is  only  found  in  one  large  phylum 
of  animals,  namely,  the  vertebrates,  and  ac- 
cordingly to  that  phylum  sea-squirts  belong. 


226  THE  ANIMAL  WORLD 

The  free  life  of  these  larvae  is  limited  to  a  few 
hours  or  days,  after  which  they  settle  down 
by  the  head,  fix  their  forehead  on  a  rock  by 
tentacles  and  proceed  to  twist  the  body  through 
an  angle  of  90°.  Meanwhile  the  same  curious 
double  change  takes  place  in  them  as  in  insect 
pupse.  Some  of  the  organs  disintegrate  and 
form  a  pulp,  whilst  others  differentiate  and 
develop  at  their  expense.  The  hollow  nervous 
system  is  concentrated  into  a  minute  knot, 
the  eye  drops  out,  the  nose  disappears,  the 
muscles  fall  to  pieces.  On  the  other  hand,  the 
throat  expands  and  its  perforations  become 
multiplied,  supported  by  bars  and  supplied 
with  a  rich  network  of  blood-vessels.  The 
larval  sensations  are  lost  and  ultimately  the 
psychical  grade  of  adult  life  is  the  very  lowest 
of  which  we  have  any  record. 

LARVAL  TRACES  IN  VERTEBRATES. — In 
higher  vertebrates  the  traces  of  such  larval 
life  are  hard  to  find.  There  is  no  such  whole- 
sale dissolution  and  reconstruction  in  them  as 
in  insects  and  sea-squirts,  but  still  even  in  man 
there  are  certain  organs  which  subserve  only 
a  temporary  purpose  in  early  life  and  disappear 
to  form  food  for  others  at  the  time  of  birth. 
For  example,  there  is  running  through  all  ver- 
tebrate animals  a  perforated  throat  or  "pharynx" 
(Fig.  8,  G),  functional  in  fishes  as  the  gill-cham- 


LIFE-HISTORIES  OF  ANIMALS 

her,  used  also  in  the  early  life  of  frogs  for  the 
same  purpose,  but  persisting  in  a  form  useless 
for  the  purpose  in  reptiles,  birds  and  mammals. 
The  primary  meaning  of  these  slits  was  to  allow 
the  passage  of  aerating  water  over  the  gills. 
They  were  breathing  organs  and  are  such  in 
fish  and  some  amphibia,  but  they  were  more 
than  this.  The  walls  of  these  gill-slits  gave 
rise,  even  in  gill-breathing  vertebrates,  to  struc- 
tures which,  uniting  together  on  each  side  of 
the  neck,  form  a  gland,  in  shape  resembling 
the  inflorescence  of  the  thyme  and  hence  known 
as  the  "thymus."  This  is  the  structure  which 
disappears  in  man  about  the  time  of  birth. 
It  is  one  of  those  larval  organs  which  we  have 
mentioned,  elaborated  from  the  bars  of  bran- 
chial basket-work,  which  seems  to  undergo 
just  such  dissolution  as  the  muscles,  glands 
and  other  larval  organs  of  an  insect  pupa.  In 
spite,  therefore,  of  the  enormous  gap  between 
the  lower  vertebrates  or  invertebrates  and 
man  there  are  still  indications  even  in  him  of 
that  larval  history  which  they  more  fully  play 
out  in  their  development. 

THE  RECAPITULATION  THEORY. — In  con- 
clusion a  word  should  be  said  on  the  question 
of  the  value  of  life-history  as  a  pedigree  docu- 
ment. Do  animals,  in  Marshall's  phrase,  climb 
up  their  genealogical  trees;  or  are  they  re- 


THE  ANIMAL  WORLD 

capitulating  their  ancestral  history  only  in 
such  a  general  way  as  to  render  any  deduction 
in  a  particular  case  untrustworthy?  The  ques- 
tion is  one  of  enormous  difficulty  and  the  con- 


Fig.  33.  —  Larva  and  pupa  of  the  minute  biting  midge," 
Ceratopogon.  This  tiny  insect  is  very  common  in  gar- 
dens, and  bites  at  sunset.  Its  larvse  (A)  live  on  the  bark 
of  oaks  and  of  other  trees,  and  are  5  mm.  in  length  when 
full  grown.  Its  pupa3  (B)  live  in  the  same  situations. 
(After  Johannsen.)  (X  15.) 

fident  affirmative  of  twenty  years  ago  is  now 
rarely  heard.  In  the  first  place,  the  study  of 
the  development  of  organs  leads  to  the  con- 
clusion that  there  is  a  broad  recapitulation  of 
past  history  in  the  development  of  the  individ- 


LIFE-HISTORIES  OF  ANIMALS 

ual.  The  heart,  for  instance,  the  ear  and  the 
eye,  even  in  man,  do  go  through  stages  of  de- 
velopment which  are  retained  by  lower  verte- 
brates, but  there  is  strong  evidence  that  the 
man  or  animal  as  a  whole  is  by  no  means 
following  the  succession  of  form  which  char- 
acterized his  or  their  predecessors.  Man  is  at 
no  time  a  fish,  an  amphibian  or  a  reptile,  as  it 
is  somewhat  crudely  put.  A  chick-embryo, 
at  four  days  old,  represents  an  organism  in- 
capable of  independent  life  ;  a  pupa  also  is 
incapable  of  free  life.  Hence  arises  the  bafHing 
result  that,  as  has  been  said,  the  older  history 
like  a  papyrus  has  received  additions  and  alter- 
ations of  a  later  date,  and  we  know  not  how 
much  of  the  altered  development  to  attribute 
to  that  added  matter.  Moreover,  one  of  the 
most  primitive  animals  in  the  world,  one  of 
the  most  important  links  in  the  chain  of  evo- 
lutionary evidence,  is  the  caterpillar-like  Peri- 
patus,  a  link  between  Annelids  and  Arthropods, 
but  the  development  of  Peripatus  is  direct  and 
sheds  no  light  whatever  on  the  Annelids  or 
Arthropods  that  it  links  up.  So  it  is  for  the 
oldest  animals  in  nearly  all  cases:  scorpions, 
lamp-shells,  tusk-shells,  Nautilus,  Ceratodus, 
primitive  insects  ;  these  are  the  aristocrats 
with  long  pedigrees,  living  fossils,  and  none 
of  them  has  a  life-history  that  throws  any  im- 


£30  THE  ANIMAL  WORLD 

portant  light  on  their  past.  Whatever  conclusion 
we  can  draw  from  animal  histories,  one,  at  any 
rate,  is  sound,  namely,  that  (in  critical  cases 
such  as  we  have  mentioned)  the  earlier  chapters 
are  entirely  missing.  The  study  of  heredity 
under  Mendel's  stimulus  shows  how  ancestral 
qualities  may  be  bred  out,  and  the  whole  trend 
of  opinion  is  at  present  to  place  the  crucial 
evidence  for  past  history  not  in  development 
and  not  in  breeding  but  in  the  records  of  the 
rocks. 


CHAPTER  X 

HEREDITY   AND   VARIATION 

BY  heredity  is  meant  the  degree  of  likeness 
between  parents  and  their  offspring.  Origi- 
nally the  term  implied  succession  in  tenure,  but 
as  in  the  case  of  so  many  scientific  words,  the 
original  significance  has  been  altered  and  the 
alteration  established  by  custom.  In  its  modern 
sense  heredity  is  a  measure  of  the  resemblance 
between  two  or  more  generations  of  the  same 
family. 

In  ordinary  speech  heredity  is  spoken  of  in 
many  illegitimate  ways.  Sometimes  it  is  re- 
garded as  a  force  by  which  the  recognizably 
close  resemblance  of  two  or  more  successive 


HEREDITY  AND  VARIATION       231 

generations  is  to  be  explained.  Frequently 
it  is  thought  of  as  a  principle  which  in  some 
unexplained  fashion  endows  father  and  son 
with  their  common  likeness.  There  is  no 
justification,  however,  for  regarding  heredity 
as  in  itself  more  than  the  expression  of  com- 
plex interactions.  The  causes  of  the  undeni- 
able likeness  are  undoubtedly  complex  and 
far  from  being  fully  understood,  but  the  ex- 
pression they  give  rise  to  in  each  generation 
of  a  family  may  be  simply  so  many  feet  high, 
such  a  colour  of  eye  or  hair,  such  a  shape  of 
hand  or  of  ear.  Heredity  is  really,  then,  a 
measure  of  the  effect  or  likeness. 

RELATION  OF  HEREDITY  TO  VARIATION. — 
When,  however,  the  causes  of  these  likenesses 
are  investigated  we  still  speak  of  the  study  of 
heredity.  In  this  deeper  aspect  of  the  prob- 
lem it  is  not  the  symphonic  expression,  the 
outcome  of  the  causes,  which  is  insisted  upon 
so  much  as  the  notes  and  phrases,  the  analysis 
of  the  composition.  We  are  no  longer  measur- 
ing the  likeness  between  sons  and  fathers,  but 
are  attempting  to  understand  why  a  father 
with  the  note  of  height  and  a  mother  without 
it,  have  children  of  a  composition  that  differs 
from  that  of  either  parent.  The  subject  passes 
into  the  analytic  phase,  and  it  is  only  on  realiz- 
ing the  complexity  of  the  composition  that 


THE  ANIMAL  WORLD 

we  recognize  its  symphonic  nature.  If  we 
compare  the  leaves  of  a  tree  with  those  of  a 
sapling  sprung  from  the  tree  we  may  be  able 
to  express  the  result  quite  simply.  The  degree 
of  likeness  is  heredity.  But  we  are  not  satisfied 
with  that  result;  we  wish  to  know  what  causes 
induce  the  degree  of  likeness.  We  find  the 
leaves  of  the  parent  tree  are  not  all  alike,  nor  are 
those  of  the  daughter  tree  ;  and  we  have  first 
to  determine  how  far  we  can  express  this  diver- 
sity between  the  leaves  of  each  tree.  This 
diversity  we  call  variation.  Variation  is  thus 
a  measure  of  the  resemblance  or  difference 
between  members  of  an  individual  or  between 
members  of  a  family.  But  heredity  implies 
genesis.  It  involves  two  successive  generations 
however  produced,  and  it  is  this  genetic  factor 
which  distinguishes  heredity  from  variation. 

Variation  is  no  special  attribute  of  life.  We 
can  speak  of  the  degree  of  resemblance  between 
varieties  of  soils,  of  hard  or  soft  water,  and  of 
many  other  objects,  though  in  a  biological 
sense  variation  is  still  conveniently  employed 
as  a  measure  of  resemblance  between  com- 
parable individuals  or  parts  of  individuals, 
whether  of  the  same  generation  or  not.  We 
can  speak  of  the  variation  of  cloths,  calicoes, 
cotton,  and  also  of  the  human  skull,  but  the 
value  of  the  observation  depends  on  the  com- 


HEREDITY  AND  VARIATION      233 

parable  nature  of  the  material,  and  when  we 
have  such  data  arranged  in  successive  generations 
then  the  variations  become  heredity  and  only 
then.  The  notion  of  genesis  of  succeeding 
families  is  essential  to  heredity.  It  is  not  es- 
sential to  variation.  There  is  a  lapse  of  time 
involved  in  heredity,  whereas  variation  is  best 
expressed  as  a  measure  of  likeness  between 
contemporaries.  The  likeness  between  brothers 
and  sisters  is  measured  by  variation;  that 
between  both  of  these  and  their  parents  is  an 
expression  of  heredity;  and  we  can  measure 
the  likeness  of  all  children  of  the  same  age,  or 
of  the  same  nation.  Some  basis  of  comparison 
there  must  be,  and  those  results  are  of  most 
value  in  which  the  material  measured  is  most 
closely  comparable;  e.  g.  the  parts  of  an  in- 
dividual or  the  individuals  of  a  family.  Before 
the  study  of  heredity  can  be  fitly  undertaken, 
therefore,  it  is  necessary  that  the  problem  of 
variation  should  be  understood,  and  this  pre- 
caution is  now  widely  recognized  since  the 
question  of  variation  underlies  the  modern 
theories  of  evolution. 

ANIMAL  CLASSIFICATION. — Before  the  prob- 
lem of  variation  in  its  biological  sense  can  be 
appreciated,  it  is  necessary  to  state  the  prin- 
ciples of  classification.  Animals  are  not  only 
organized  structurally,  they  are  also  organized 


234  THE  ANIMAL  WORLD 

in  groups  subordinate  to  more  comprehensive 
groups:  they  are  segregated  into  battalions 
and  companies.  Man,  for  example,  is  a  large 
division,  composed  of  many  races,  and  usually 
regarded  as  forming  a  single  species  made  up 
of  these  geographical  races.  Herrings,  par- 
tridges, cockles  and  many  other  widely  spread 
animals  are  likewise  composed,  not  only  of  so 
many  male  and  female  individuals,  but  of  these 
grouped  in  races  so  that  an  expert  can  place  an 
individual  roughly  in  the  variety  to  which  it 
belongs,  and  can  often  say  from  what  part  of 
the  country  it  has  come.  Any  widely  distributed 
animal  is  found  to  fall  into  a  certain  number 
of  these  geographical  varieties,  and  it  is  only 
by  lumping  these  races  together  that  we  get 
the  idea  of  a  species.  Such  lumping  may  even 
go  beyond  its  legitimate  bounds.  All  dogs, 
for  example,  and  all  horses  are  supposed  to 
form  a  single  species  in  each  case,  but  such  an 
expression  leaves  out  of  sight  the  fact  that  we 
are  attempting  to  impose  upon  a  cross-bred 
animal  the  terminology  of  a  pure-bred  one. 
Dogs  are  probably  descended  from  more  than 
one  wild  species.  Horses  show  many  indi- 
cations of  an  equally  mixed  descent.  It  is 
better,  therefore,  to  begin  by  recognizing  the 
the  racial  characters  before  attempting  a  syn- 
thesis such  as  is  implied  in  the  word  species. 


HEREDITY  AND  VARIATION      £35 

The  study  of  variation,  therefore,  has  to 
reckon  with  the  tendency  of  a  widely  distri- 
buted animal,  not  only  to  show  individual  differ- 
ences between  members  of  contemporary  and 
of  earlier  generations,  but  also  to  vary  in  a 
definite  manner  over  a  certain  district  or  under 
certain  conditions.  In  a  word,  variation  deals 
with  individuals  and  with  geographical  races. 
For  example,  we  can  measure  the  height  of 
the  men  in  an  army,  but  the  results  will  be  of 
little  value  if  the  army  is  of  mixed  origin.  To 
compare  individuals  effectively  we  ought  to 
know  that  they  have  a  certain  relationship  to 
one  another,  and  the  closer  the  relationship 
the  more  valuable  are  the  results.  All  the  men 
of  a  race  would  in  a  certain  sense  offer  compa- 
rable material,  all  the  men  of  a  nation  would 
constitute  a  still  better  foundation  for  useful 
results,  but  to  use  them  to  the  best  advantage 
it  would  be  necessary  to  have  data  as  to  age, 
up-bringing,  and  so  on. 

VARIATION  IN  NATURE. — Having  realized 
the  necessity  for  precaution  in  obtaining  com- 
parable data  for  the  study  of  variation,  a  few 
words  may  be  said  about  the  kinds  of  variation 
which  occur  in  nature.  Every  one  realizes 
individual  differences  in  man,  and  those  who 
are  engaged  in  agriculture  realize  an  individ- 
uality in  cattle,  sheep  and  dogs,  even  when, 


£36  THE  ANIMAL  WORLD 

to  the  inexperienced  observer,  the  members 
of  a  flock,  herd  or  pack  may  look  almost  exactly 
alike.  Increased  familiarity  with  animal  life 
shows  that  this  phenomenon  is  not  limited 
to  man  and  domesticated  animals.  The  mem- 
bers of  wild  animals'  families  show  individual 
differences  though  we  are  less  acquainted  with 
them.  A  sweep  of  a  shrimp-net  over  the  sea- 
weeds on  a  rocky  coast  will  disclose  a  number 
of  specimens  of  Hippolyte  varians,  hardly  two 
of  which  are  alike;  some  are  green,  some  red, 
some  brown,  some  are  spotted  with  blue,  other 
are  not,  some  are  uniformly  tinted,  other  are 
marbled  or  lined  with  colours.  We  do  not  know, 
however,  whether  these  casually  collected  speci- 
mens belong  to  two  or  more  families.  But 
even  if  we  take  the  offspring  of  a  single  individ- 
ual, say  a  green  specimen,  we  find  that  some 
members  of  the  brood  are  different  in  colour 
from  the  rest,  some  are  harder,  some  have  the 
rostrum  (that  is,  the  pointed  spine  or  nose  in 
front  of  the  head)  with  so  many  notches,  others 
have  fewer,  and  so  on.  Still  better  is  the  case 
of  the  eggs  of  the  common  house-sparrow.  With 
most  British  birds  the  eggs  of  a  single  clutch 
are  apparently  exactly  alike  in  shape,  size,  weight 
and  colouring.  In  the  sparrow,  however,  the 
eggs  differ  among  themselves  to  an  extraordinary 
degree:  still  more  do_they  differ  when  several 


HEREDITY  AND  VARIATION      237 

clutches  are  compared.  In  just  such  a  way 
is  there  individual  variation  of  greater  and  of 
lesser  extent  amongst  animals  generally. 

DEPARTURE  FROM  SYMMETRY. — But  the  nature 
of  variation  is  more  deep-seated  than  individual 
or  racial  differences.  Regarding  the  individual 
as  an  organization,  as  a  collection  and  synthesis 
of  members,  we  have  departures  from  symmetry 
brought  about  by  one  or  more  members  develop- 
ing more  strongly  than  their  fellows.  For  exam- 
ple, all  animals  that  creep,  walk  or  fly  are  bilater- 
ally symmetrical,  they  have  right  and  left  sides, 
and  we  unconsciously  assume  that  the  sides 
ought  to  be  alike  or  rather  complementary.  A 
tailor,  however,  knows  that  our  shoulders  are 
not  symmetrical,  and  that  usually  the  right 
side  is  either  more  strongly  developed  or  more 
rounded  than  the  left.  Our  eyes  look  alike, 
but  the  eyebrows  are  often  markedly  different 
in  behaviour,  and  an  optician  readily  discovers 
the  inequality  in  strength  of  the  left  or  right 
eye  in  many  people.  The  hands  are  always 
different  in  their  capacity  for  drawing,  writing, 
exertion,  indicating  that  the  brain  is  not  truly 
symmetrical.  In  fact,  all  the  evidence  goes 
to  show  that  the  two  sides  of  our  body  vary 
independently. 

INFLUENCE  OF  EXTERNAL  CONDITIONS. — But 
not  only  are  there  individual  variations  which  ex- 


238  THE  ANIMAL  WORLD 

press  inborn  differences  between  members  of  the 
same  family  or  of  the  same  body.  In  addition 
the  influence  of  altered  conditions,  both  external 
and  internal,  modify  an  individual  and  produce 
an  effect  in  which  the  shares  due  respectively  to 
nature  and  to  nurture  are  hard  to  estimate. 
Take,  for  instance,  the  influence  of  meat.  Long 
ago  John  Hunter,  the  famous  anatomist,  fed  gulls 
for  a  year  with  a  diet  of  grain  in  place  of  their 
usual  food  of  fish  and  soft  food.  The  stomach 
of  these  birds  developed  a  more  muscular  coat 
and  its  lining  assumed  the  structure  of  a  gizzard. 
More  remarkable  still  is  the  annual  change  of 
structure  that  occurs  in  the  same  gull  according 
to  Dr.  Edmonstone.  In  certain  districts  these 
birds  feed  part  of  the  year  on  fish  and  during 
the  rest  on  grain.  During  the  period  of  the 
fish  diet,  the  stomach  assumes  a  soft  structure 
characteristic  of  the  bird  in  most  districts 
throughout  the  year,  but  when  the  gulls  take 
to  the  fields  the  lining  of  the  stomach  ac- 
quires a  gizzard-like  appearance,  which  is  again 
replaced  by  the  less  muscular  coat  and  softer 
lining  when  the  gulls  have  exchanged  the  fields 
for  coastal  life. 

INFLUENCE  OF  LIGHT  ON  FLAT-FISH. — Again, 
the  lower  white  side  of  flat-fish  (Flounder)  can 
in  the  course  of  a  year  be  made  more  or  less  pig- 
mented  by  exposing  it  to  light  reflected  from 


HEREDITY  AND  VARIATION      239 

below  instead  of  from  above:  and  it  is  not  an 
uncommon  thing  to  find  "ambicolorate"  floun- 
ders or  plaice  in  nature;  that  is  to  say,  speci- 
mens which  are  coloured  on  both  sides.  In  this 
case,  however,  we  do  not  know  that  the  habits 
of  these  fish  are  such  as  to  expose  their  under 
surface  to  an  especial  amount  of  light. 

A  more  indisputable  natural  experiment  is 
the  one  which  has  resulted  in  the  remarkable 
coloration  of  Echeneis,  a  fish  often  attached  by 
its  head  to  ships  or  sharks.  The  dorsal  fin  of 
this  fish  is  modified  to  form  a  sucker  by  the  aid 
of  which  it  often  lies  inverted,  its  upper  surface 
being  turned  downwards,  its  lower  surface  up- 
wards. The  resulting  coloration  is  exactly  the 
converse  of  what  usually  obtains,  for  such  fish 
are  light  above  and  dark  below.  We  have,  then, 
variations  brought  about  by  change  of  conditions 
and  all  recent  experimental  work  goes  to  show 
that  plants  and  animals  are  susceptible  of  far- 
reaching  changes  in  appearance  and  structure, 
and  even  in  modes  of  reproduction,  when  sub- 
mitted for  one  or  more  generations  to  the  influ- 
ence of  an  environment  different  from  that  in 
which  they  normally  develop.  Here,  again,  we 
are  struck  by  the  reserves  of  capacity  in  animal 
life.  Individual  life  appears  to  be  only  a  partial 
expression  of  its  total  capacity  for  assuming 
form  or  structure  and  of  initiating  developmental 
changes  in  offspring. 


240  THE  ANIMAL  WORLD 

VARIATION  DUE  TO  INTERNAL  CAUSES. — Vari- 
ation, however,  not  only  arises  in  connection  with 
external  changes.  It  may  also  be  induced  by  in- 
ternal ones.  We  are  coming  to  realize  that  every 
animal  is  not  only  a  constellation  of  cells  arranged 
in  tissues,  but  that  the  crevices  of  this  system  are 
filled  with  a  fluid  medium,  blood  or  lymph,  pos- 
sessing a  surprising  richness  of  capacity.  Bath- 
ing as  it  does  every  element  (or  "cell")  in  this 
constellation,  this  "milieu  interne"  is  a  sort  of 
inland  sea  washing  the  shores  of  susceptible  and 
ever-changing  islets  which  are  absorbing  nourish- 
ment from  its  waters  and  discharging  waste  into 
them.  This  inland  sea  is  the  complement  of  the 
environment.  Its  waters  flow  and  ebb,  bearing 
food  and  oxygen,  like  the  outer  sea;  and  just  as 
when  we  modify  the  quality  of  sea-water,  animal 
life  feels  the  change  for  good  or  for  ill,  so  when  the 
internal  sea  is  altered  the  body  responds  as  by 
fever  or  lethargy,  by  increased  or  by  retrograde 
development.  These  two  seas,  in  fact,  act  upon 
each  other;  the  environment  produces  its  effect 
upon  animals  largely  through  acting  in  the  first 
instance  upon  this  inner  sea,  which  in  turn  affects 
the  islands  of  cells  and  the  archipelagoes  of  nerve 
and  muscle.  Change  of  environment,  then, 
works  chiefly  indirectly  upon  the  body  through 
the  medium  of  the  inner  environment. 

FACTORS  GOVERNING  THE  INTERNAL  MEDIUM. 


HEREDITY  AND  VARIATION      241 

— But  this  internal  medium  has  its  own  regula- 
tions and  governance.  It  is  specific  for  each 
kind  of  animal  or  at  least  for  the  majority.  The 
phrase,  blood-relationship  expresses  more  than 
was  realized  in  the  minds  of  those  who  coined  it. 
By  the  blood  alone  the  relationship  of  an  animal 
can  be  determined  even  down  to  the  particular 
species  to  which  it  belongs.  Such  facts  show  that 
there  must  exist  a  mechanism  for  creating  and 
maintaining  this  constancy.  The  medium  itself 
is  of  a  great  complexity,  recalling  again  the  sea 
of  which  it  is  the  analogue.  Some  of  the  sub- 
stances in  it  are  even  the  same  as  those  which 
occur  in  the  sea,  such,  for  instance,  as  common 
salt.  Indeed,  amongst  lower  aquatic  animals 
there  may  be  an  exchange  of  substance  directly 
between  the  outer  and  the  inner  medium,  but  in 
the  higher  animals  the  blood  is  only  indirectly 
related  and  to  a  slight  extent  with  the  food,  air 
or  water  of  the  outer  medium.  Its  composition 
is  governed  largely  by  special  "glands"  which 
extract  substances  from  the  incoming  blood,  work 
them  into  new  combinations  and  with  due  re- 
serve discharge  them  into  the  outgoing  blood- 
stream, from  whence  they  find  their  way  into  the 
lymph  and  so  to  the  very  crevices  of  the  organism. 
Such  glands,  for  example,  as  the  thyroid,  thy- 
mus,  suprarenals,  are  employed  in  the  work  of 
regulating  the  composition  of  this  shallow  sea, 


THE  ANIMAL  WORLD 

and  it  seems  probable  that  many  organs,  if  not, 
indeed,  all  the  tissues,  contribute  to  the  final 
result.  But  there  are  some  which  contribute 
more  largely  than  others,  and  change  in  these 
brings  about  a  marked  alteration  in  the  feeding 
power  and  other  properties  of  the  medium  upon 
which  the  whole  body  depends  for  the  continuance 
of  its  healthy,  normal  existence  and  growth.  For 
example  the  thyroid  exerts  a  far-reaching  influ- 
ence of  this  kind  and  though  it  varies  consider- 
ably in  man  without  producing  any  ill  effects, 
such  variation  has  a  limit;  and  if  the  thyroid 
effluent  is  either  insufficient  in  quantity  or  de- 
fective in  quality,  the  whole  body  suffers  from 
starvation  just  as  real  as  if  food  of  the  ordinary 
kind  were  withheld.  In  a  child  with  imperfect 
thyroid  the  body  becomes  stunted  and  growth 
ceases,  the  mental  faculties  may  be  arrested  and 
feeble-mindedness  ensue;  and  the  trouble  may 
hang  out  a  signal  of  distress  by  the  assumption 
of  goitre  or  may  lie  concealed. 

Variation,  then,  is  caused  by  internal  as  well 
as  by  external  changes.  So  strongly  indeed  do 
we  become  impressed  by  the  facilities  for  altera- 
tion in  the  external  world  and  in  the  internal 
medium  that  we  begin  to  wonder  how  the  con- 
stancy that  we  all  admit,  the  average  size,  shape, 
structure  and  behaviour  of  each  species  of  animal, 
is  maintained.  We  begin  to  perceive  the  neces- 


HEREDITY  AND  VARIATION      243 

sity  for  a  regulative  machinery  which  must  so  in- 
fluence or  counteract  these  tendencies  to  vary  as 
to  allow  of  only  a  comparatively  slight  degree  of 
divergence  from  the  standard  of  each  animal 
kind.  Such  governance,  however,  is  at  present 
far  beyond  our  ken. 

THE  PROBLEMS  OF  HEREDITY. — Having  now 
realized  to  some  extent  that  the  orderliness  of 
nature,  the  sharp  lines  that  demarcate  the  animal 
world  into  battalions  and  companies,  genera  and 
species,  is  a  balance  struck  between  tendencies  to 
vary  (centrifugal  tendencies)  and  tendencies  to 
typify  (centripetal  tendencies),  we  come  back  to 
the  problems  of  heredity  which  as  we  have  defined 
them  are  the  relations  between  successive  gener- 
ations. The  nature  of  these  problems  will  be- 
come clearer  if  we  take  an  example  of  the  kind 
of  material  with  which  heredity  deals.  The  enor- 
mously complex  nature  of  human  inheritance  ren- 
ders a  simpler  example  necessary. 

DIVISION  OF  CELLS. — Amongst  protozoa  the 
process  of  fission  or  division  leads  to  the  multi- 
plication of  isolated  cells  whereas  in  higher  ani- 
mals the  cells  cohere  and  form  tissues.  Each 
time  that  a  Paramecium  divides,  the  body  of  the 
parent  is  shared  by  the  daughter-cells,  and  the 
likeness  of  daughters  to  one  another  and  to  their 
parents  is  largely  due  to  the  mere  slicing  of  the 
latter  in  two,  accompanied,  however,  by  other 


244  THE  ANIMAL  WORLD 

changes  which  are  also  only|a  dividing  of  sub- 
stance into  equal  halves.  Such  dividing  cells  are 
like  the  cells  of  the  animal  body,  they  are  somatic 
cells;  and  just  as  our  hair  grows  by  multiplication 
of  cells  at  the  hair-follicles  so  the  Paramecium 
grows  even  though  its  cells  fall  apart  from  one 
another.  Fission  is  simply  a  form  of  growth. 

HEREDITY  is  THE  STUDY  OF  SYMMETRICAL 
DIVISIONS. —  There  is,  however,  one  important 
difference  between  the  division  of  a  Paramecium 
and  that  of  a  tissue-cell.  In  Paramecium  the  two 
daughter-cells  have  each  to  complete  a  portion 
of  their  bodies  by  growth.  One  has  to  make  a  new 
"tail,"  the  other  a  new  "head,"  whereas  when 
two  tissue-cells  are  formed  by  division  of  one, 
each  of  the  daughter-cells  has  no  such  "ends"  to 
form.  Paramecium  produces  a  succession  of  new 
individuals  which  are  not  optical  images.  The  like 
parts  are  alternate  in  Paramecium  and  adjacent  is 
a  tissue-cell.  The  symmetry  is  also  different  in 
the  two  cases:  the  daughter-cells  of  Paramecium 
are  related  to  one  another  like  the  right  and  left 
hands,  daughter-cells  of  a  tissue-element  are 
mirror-images  of  each  other.  Viewed  from  this 
aspect,  heredity  is  a  study  of  such  symmetrical 
divisions.  (Fig.  34.  See  Bateson's  work,  p.  256.) 

VARIATION,  THE  STUDY  OF  ASYMMETRICAL 
DIVISIONS. — Now  suppose  that  the  plane  of  divi- 
sion does  not  pass  exactly  transversely  to  the 


HEREDITY  AND  VARIATION      245 

body  of  the  parent  Paramecium,  but,  as  some- 
times happens,  cuts  it  irregularly  so  as  to  leave 
one  daughter-cell  with  a  process  and  the  other 
with  an  indentation.  The  symmetry  of  the  two 


Fig.  34.  —  To  illustrate  the  relation  of  heredity  to  symmetry. 

A  dividing  Paramecium,  showing  the  two  mouths  (M,  M'),' 
the  anterior  end  and  the  posterior  end  of  the  two  daugh- 
ter-cells. The  two  nuclei  are  also  in  the  act  of  dividing. 
The  symmetry  of  the  two  daughter-Paramecia  is 
really  not  complete.  The  anterior  has  to  regenerate  a 
tail-end,  the  posterior  has  to  re-form  its  head-end. 

is  now  no  longer  what  it  was,  and  two  variations 
have  been  produced.  Variation  then  is  asym- 
metrical division,  and  from  such  a  standpoint  the 
separation  of  tissues  in  a  dividing  egg  begins  with 
the  first  asymmetrical  division.  (Fig.  35.) 
NATURE  OF  GAMETES. — The  case  of  Parame* 


246  THE  ANIMAL  WORLD 

cium  is,  however,  simplified  by  the  fact  that  the 
body  of  the  parent  is  merged  in  that  of  the  chil- 
dren. When  conjugation  occurs  (p.  21)  there  is 
an  interchange  of  the  two  cells,  so  that  when  they 
separate  and  begin  to  divide,  each  carries  away 
with  it  some  portion  of  the  other.  The  daughter- 
cells  of  any  Paramecium,  therefore,  are  derived 
not  merely  from  one  but  from  two  parent  cells 
and  each  of  these  from  two  and  so  on.  The  min- 
gling of  two  parents  is  accompanied  by  complex 
changes,  and  the  two  are  termed  gametes.  It  re- 
sults from  this  consideration  that  the  children 
contain  contributions  from  both  parents  (the 
preliminary  process  resulting  in  the  casting  out  of 
much  previously  received  ancestral  contribution), 
and  are,  therefore,  essentially  double  structures  if 
we  regard  the  parents  as  each  a  single  one.  In 
most  cases  the  gametes  do  not  separate  after  con- 
jugation, but  remain  fused  together.  Such  a 
fused  mass  is  termed  a  zygote  and  will  presently 
divide  into  cells  and  form  a  fresh  individual. 

DOUBLE  NATURE  OF  ZYGOTE. — In  higher  ani- 
mals the  same  process  usually  occurs.  The  egg 
or  zygote  is  a  double  structure  derived  from  both 
parents.  It  divides,  but  the  products  are  at  first 
mirror  images  of  one  another  and  remain  united. 
Presently  there  comes  a  division  which  cuts  the 
egg  into  right  and  left  halves,  the  foundation  of 
the  future  sides,  and  forms  the  basis  of  a  new  type 


HEREDITY  AND  VARIATION      247 

of  symmetry.  Lastly  the  cell-mass  becomes  dif- 
ferentiated. That  is  variation  and  is  essentially 
due  to  asymmetrical  division.  The  study  of 
heredity  is  the  study  of  these  variations,  but  we 
cannot  often  trace  the  obvious  characters  which 
distinguish  grown  individuals  such  as  hairiness, 
height,  colour,  and  so  on,  to  that  point  in  devel- 
opment at  which  the  asymmetry  first  began.  If 
we  could,  an  enormous  advance  would  be  possible 
in  the  study  of  heredity. 

MENDELISM. — In  place  of  such  intensive  study, 
heredity  at  present  deals  with  immensely  complex 
symphonic  expressions:  and  has  made  such  ad- 
vances that  by  its  aid  a  correct  result  can  be  fore- 
seen even  when  the  particular  "cross"  has  not 
been  made  before.  This  result  has  been  rendered 
possible  by  the  discovery,  remarkable  alike  for  its 
profound  nature  and  the  neglect  it  met  with,  that 
on  crossing  contrasted  sweet  peas,  the  offspring 
display  in  their  characters  a  simple  multiple  re- 
lation, the  parental  characters  appearing  in  the 
first  or  (on  self-fertilization)  in  the  second  cross 
not  mixed  or  averaged  but  segregated  out  in  a 
definite  proportion.  Thus  a  tall  pea  (T)  crossed 
with  the  pollen  of  a  short  pea  (t)  gives  all  tall  in 
the  first  generation,  and  when  these  are  self- 
fertilized  their  offspring  are  tall  and  short  in  the 
proportion  of  three  tall  peas  to  one  short.  On 
repeating  the  process,  the  tall  peas  are  seen  to  be- 


248          ^THE   ANIMAL   WORLD 

have  in  one  of  two  ways.  Some  (TT)  yield  tall 
plants  only,  others  (Tt)  tall  and  short,  again  in  the 
proportion  of  three  tall  to  one  short,  whilst  the 
short  or  bush  peas  (tt)  yield  short  only.  In  this 
way  the  pure  lines  are  extracted  from  the  impure, 
or,  to  use  correct  language,  the  homozygotes  are 
separated  from  the  heterozygotes. 

FACTORS  IN  HEREDITY. — In  such  a  way  the 
factors  for  tallness  and  for  shortness  are  seen  to 
be  definite  transmissible  qualities  even  if  we 
know  nothing  about  their  mode  of  action.  These 
factors  are  pure  in  certain  offspring  (homozygotes) 
mingled  in  others  (heterozygotes);  but  when 
mingled  they  can  be  separated  out  by  breeding, 
though  never  perfectly — a  certain  number  of  the 
mingled  strain  always  reappears.  The  important 
points  to  notice  are  the  purity  of  the  "extracted" 
strains  and  the  simple  proportion  they  bear  to 
the  mingled  strain.  Thus,  if  there  be  2  n  hetero- 
zygotes there  will  be  n  tall  and  n  short  plants  of 
the  pure  strains. 

This  result,  the  extracting  or  segregating  out 
of  plants  with  a  certain  property  (such  as  the 
property  of  growth  to  6  feet  or  of  growth  to  only 
2  feet)  from  a  mixed  race  is  the  central  feature 
of  the  great  discovery  made  by  Gregor  Mendel, 
in  the  monastery  at  Briinn,  in  Bohemia,  and 
published  in  1866.  If  we  use  the  term  allelo- 
morphs for  those  alternative  characters  (such  as 


HEREDITY  AND  VARIATION         249 

tallness  and  shortness  of  habit,  yellowness  and 
greenness  of  seed)  then  we  can  say  that  when 
the  allelomorphs  segregate  out,  the  homozygous 
offspring  breed  true  to  the  character  in  question, 
irrespective  of  their  ancestry. 


Fig.  35.  —  Showing  the  division  of  an  animal  ogg-coll  into 
like  parts  (optical  images)  (heredity)  and  (D)  into  upper 
and  lower  layers,  which  are  unlike  (variation). 

The  two  cells  (B)  or  four  cells  (C),  if  shaken  apart, 
will  each  develop  into  a  perfect  little  fish,  but  a  half 
and  a  quarter  respectively  the  size  of  a  fish  developed 
from  (A). 

The  eight  cells  (D)  will  not  develop  into  a  fish  if 
shaken  apart,  but  will  die.  A  qualitative  division  has 
taken  place  along  the  horizontal  line,  cleaving  the  four 
cells  into  eight.  That  is  variation. 


But  Mendel's  discovery  went  farther  than  this. 
Besides  a  single  pair  of  alternative  characters 
there  are  in  peas  and  in  animals  several  such 
pairs  of  allelomorphs,  the  colours  of  the  flowers 
for  example,  and  it  was  in  connection  with  these 
that  Mendel  made  his  further  advance.  By 


B. 


Fig.  36. 

THE  BIRTH  OF  A  SEA  URCHIN:     to  illustrate  the  struggle 
between  larval  and  adult  tissues  (see  p.  214).     (  X  45.) 

A.  The   free-swimming   larva    called   Pluteus,    from   its    re- 

semblance to  a  painter's  easel  (inverted).  The  body  is 
drawn  out  into  ciliated  arms  (AR) ,  and  is  provided  with  a 
larval  mouth  (ML),  a  food-tube  (ST),  and  a  brain  with 
two  eye-spots  (BR).  On  the  left  side  of  the  larva  is 
the  first  trace  of  the  future  Echinus  (EC). 

B.  The  struggle  showing  the  Echinus  victorious  over  the 

larva.  The  latter  has  now  shrunk,  and  the  Echinus 
has  developed  at  its  expense.  MA,  mouth  of  the  Echinus. 
NS,  nervous  system  of  the  Echinus.  Spines  and  suckers 
are  developing  on  the  body  of  the  latter.  (After  Mac- 
Bride,  slightly  modified.) 
250 


HEREDITY  AND  VARIATION        251 

crossing  tall  purple  peas  with  short  white  ones, 
the  crossed  individuals  gave  rise,  in  the  second 
generation,  to  short  purple  and  tall  white  peas  as 
well  as  to  the  original  forms.  That  is  to  say, 
Mendel  found  that  a  definite  proportion  of  new 
pure  combination  are  produced.  Thus  organic 
characters  of  this  type  appear  as  "factors  which 
can  be  replaced  by  alternative  characters  without 
otherwise  altering  the  constitution  of  the  organ- 
ism" (Doncaster):  and  the  result  of  such  knowl- 
edge is  seen  to-day  in  the  raising  of  a  new  form 
of  wheat,  which  combines  the  valuable  quality 
of  a  delicate  race  with  the  resisting  power  against 
disease  of  a  less  valuable  one. 

Now  in  animals  the  discovery  of  "factors," 
or  alternative  characters,  which  behave  in  this 
way  has  made  much  progress.  Recombinations 
can  be  made  in  animals  as  in  the  case  of  the  sweet 
pea.  Thus  in  the  guinea-pig,  starting  from  an 
albino,  smooth  coat,  long  hair,  and  crossing  with 
a  coloured,  rough-coated,  short-haired  specimen, 
the  first  generation  is  coloured,  with  rough  coat 
and  short  hair.  These  crossed  together  give 
various  recombinations  such  as  albino,  rough 
coat,  short  hair;  coloured,  smooth  coat,  long 
hair.  By  selecting  these  any  combination  can 
be  fixed,  and  after  experience  has  been  gained 
the  operation  may  be  guided  with  certainty. 

Natural  inheritance  is,  however,  far  more  com- 


THE  ANIMAL  WORLD 

plex  than  this,  and  the  knowledge  of  how  pairs 
of  characters  may  react  upon  one  another  so  as 
to  give  a  combined  effect — so  far  studied  chiefly 
in  the  colours  of  animals — is  sufficient  to  lead  to 
the  conclusion  that  the  general  appearance  of 
an  animal  is  no  guide  to  its  real  constitution,  but 
that  this  constitution  can  in  many  cases  be 
determined  by  principles  which  are  only  now 
becoming  fully  appreciated. 


BIBLIOGRAPHY 

Text-books  (General). 

T.  J.  PARKER  and  W.  A.  HASWELL. — "Text-book  of  Zoology," 
2  vols.  The  Macmillan  Co.,  New  York,  1910. 

J.  ARTHUR  THOMSON. — "Outlines  of  Zoology."  Hodder  & 
Stoughton,  London,  1910. 

F.  W.  GAMBLE. — "Animal  Life."  E.  P.  Button  &  Co., 
New  York,  1908. 

MARSHALL  and  HURST. — "Practical  Zoology."  G.  P.  Put- 
nam's Sons,  New  York,  1906. 

Animal  Behaviour. 
(  Lo  comotion) — 
MAREY. — "Animal  Mechanism."     International  Scientific 

Series,  vol.  xi.    Paul,  Trench,  Triibner  &  Co.,  London. 
PETTIGREW. — "Animal  Locomotion,"  Ibid.  vol.  vii. 
(Senses  and  Intelligence) — 

AVEBURY. — International  Scientific  Series,  vol.  65. 
PECKHAM. — "Instincts    and    Habits   of    Solitary   Wasps." 

Houghton,  Mifflin  Co.,  Boston,  1905. 
FABRE. — "  Souvenirs  Entomologiques."    Paris,  1879-1891. 

Animal  Coloration. 

A.  R.  WALLACE. — "Darwinism."  The  Macmillan  Co.,  New 
York. 

E.  B.  POULTON. — "Colours  of  Animals,"  International  Scien- 

tific Series,  vol.  68. 

M.  NEWBIGIN. — "Colour  in  Nature."    Murray,  1898. 
THAYER. — "Animal  Coloration."     The  Macmillan  Co.,  New 

York,  1910. 

Animal  Distribution. 

F.  E.  BEDDARD. — Cambridge  Natural  Science  Manuals.    Mac- 

millan, New  York,  1895. 

A.  HEILPRIN. — International  Scientific  Series.  D.  Appleton 
&  Co.,  New  York,  1887. 

253 


254  BIBLIOGRAPHY 

Variation. 

C.  DARWIN. — "  Variation  under  Domestication."     Appleton^ 

New  York. 
H.  M.  VERNON. — "Variation  in  Animals  and  Plants."   Henry 

Holt  &  Co.,  New  York,  1903. 

Heredity. 

W.  BATESON. — Mendel's  "Principles  of  Heredity."    Putnam, 

New  York,  1908. 
L.  DONCASTEB. — "Heredity."    Putnam,  New  York,  1910.  J 


GLOSSARY— INDEX 


ACCELOMATA,  animals  without  a 
coelom  (Planarians,  etc.),  30 

ADOLESCENCE,  youth 

AIR-BLADDER,  115 

AMCEBA,  16,  208 

AMPHIBIA,  53-55 

ANALYSIS,  the  breakdown  of  a 
complexity  into  its  components 

ANNELIDS,  worms  composed  of  ring- 
like  segments,  39 

ARTHROPODS,  jointed-1  egged  ani- 
mals with  a  hard  shell,  34,  40, 
102 

BEES,  197,  201-205 
BLOOD  of  Annelids,  102:  of  Arthro- 
pods, 105 
BREATHING,  88-120 

CELLS,  the  microscopic  building- 
stones  of  the  body,  15 

CILIA  ( ' '  eye-lashes  ") ,  microscopic 
hair-like  outgrowths 

CLASSIFICATION,  39,  233 

CLEAVAGE,  subdivision  of  one  into 
two  or  more 

CCELENTERATE,  hollow  animals 
(zoophytes,  jellyfish,  etc.),  the 
cavity  of  which  is  equivalent  to 
both  the  ccelom  and  the  enteron 
(or  gut)  of  higher  animals,  27,  28 

CCELOM  (/colXos,  hollow),  the  hollow 
organ  which  lies  between  the 
skin  and  the  gut.  Its  walls  give 
rise  to  the  kidneys,  the  repro- 
ductive cells  and  other  tissues 

CCELOMATA,  the  subdivision  of  the 
animal  kingdom  which  possesses 
a  ccelom,  33-39 

COLOURS,  122-140 

COMMENSALISM,  messmates,  165 

COPEPODA,  small  shrimp-like  ani- 
mals with  rowing  feet 

CONJUGATION,  the  union  of  two 
reproductive  cells 

255 


CONSTELLATION,    the   grouping   of 

cells  to  form  a  body,  24 
CONVOLUTA,  157-163 
CORONAL,  a  crown 
CRUSTACEA,     a     division    of     the 

Arthropods       including       crabs, 

shrimps,  etc.,  107-109 
CULTURE,  a  pure  strain  or  variety 

ECHINODERMS,  spiny-skinned  ani- 
mals, including  starfish,  sea- 
urchins,  etc. 

ECTODERM,  the  outer  skin  or  layer, 
27 

EGGS,  184-201 

ENDODERM,  the  inner  skin  or  gut- 
lining,  27 

ENVIRONMENT  (surroundings) 

FACTORS,  the  conditions  upon  whicb 
a  result  follows 

FERMENTS,  76,  102 

FISH,     34;      movements     of,     51; 

t,  breathing  of,  111-117;  eggs  and 
nests  of,  191;  shoals  of,  171 

FISSION,  see  CLEAVAGE 

FLAGELLA,  whip-like  outgrowths  of 
cells 

FLAGELLATES,  a  division  of  Pro- 
tozoa, 19 

FLAT-WORMS,  see  PLANARIANS 

FOOD,  72-86 

GAMETES,  mature  reproductive  cell, 
35 

GERM-GLANDS,  the  tissues  of  re- 
production, 33 

HEREDITY,  232-254 

HOMOIOTHERM,  of  uniform  tem- 
perature 

HYDROIDS,  animals  like  Hydra,  so 
called  from  their  faculty  of  grow- 
ing afresh  when  cut  down 


256 


GLOSSARY— INDEX 


INFUSORIA,  22 

INNATE,  inborn 

INQUILINES,  resident  aliens,  165 

INSECTS,  40;  movements  of,  52; 
foodof,?82-84;  breathing  of,  104- 
106;  senses  of,  155;  guests  of, 
174-175;  care  of  young  of,  173- 
204;  life-histories  of,  205-230 

LARVA,  the  first  stage  of  free  life  in 
animals  that  afterwards  undergo 
a  great  change,  30 

LIFE-HISTORIES — of  Protozoa,  209- 
210;  of  Hydroids,  211-212;  of 
Echinoderms,  213-215;  of  Worms, 
215-216;  of  Molluscs,  215,  217; 
of  Insects,  218-229;  of  Ascidiana 
(sea-squirts),  227 

LONGEVITY,  30,  72 

MAMMALIA,  42;  movements  of,  59- 
1,70;  food  of,  70,  87;  breathing  of, 
JP121;  colouring  of,  124 

MESENCHYME,  the  middle  jelly:  a 
soft  cellular  tissue  between  the 
skin  and  the  gut,  28 

METABOLISM,  the  changes  in  living 
substance,  72 

METAZOA,  animals  composed  of 
tissues,  24,  38 

MIGRATION,  68 

MOLLUSCS,  40;  food  of,  77;  breath- 
ing of,  96-103;  eggs  of,  191,  217 

MONAD,  a  minute  Protozoon,  15 

MOVEMENTS,  43-71 

MUD-FISH,  45 

NEMERTINES,  a  group  of  long  unseg- 

mented  worms 
NUCLEUS,  the  governing  centre  of  a 

cell,  16,  19,  22 

ORGANISM,  an  individual  animal  or 

plant,  24 
OVUM,  the  ogg-cell 


PALOLO-WORM,  180-186 
PARAMECIUM,  22,  245-249 
PERCEPTION,    knowledge    obtained 

directly  by  the  senses 
PHYLUM,  a  main  stem  of  the  animal 

kingdom 

PLANARIANS,  32-33 
PLASTICITY,  ability  to  change  form 

or  function 
PROTOZOA,  14-21;    life-histories  of, 

209-210 
PSEUDOPODIA,  irregular  processes  of 

a  cell,  16,  19 

REPTILIA,  54-58 

SEA-ANEMONE,  28 

SEGMENTATION,  the  division  of  the 

body  into  comparable  transverse 

portions 
SENSATION,  the  dim  feeling  aroused 

by  the  senses 
SENSES,  145-157 
SPECIES,  an  assemblage  of  similar 

animals    or     plants    united    by 

common     racial     structure    and 

not  breeding  with  others 
SPONGES,  39 
SPORULATION,     the    formation    of 

spores 
SYMBIOSIS,     partnership     between 

alien  organisms,  155-157 
SYNTHESIS,    the    production    of    a 

complex  result  or  substance  from 

Its  simpler  components 

VARIATION,  232-250 

VORTICELLA,  23 

ZOOPHYTES,  plant-like  animals,  27 
ZYGOTE,  the  united  male  and  female 

reproductive  cells;    the  fertilized 

egg,  246 


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